The Parsha opens by relating that Hashem commanded Moshe to talk (אמר) to the Kohanim, meaning the sons of Aharon, and to tell them (ואמרת אלהם) certain details concerning the laws of becoming טמא, ritually impure, which apply specifically to them (ויקרא כ"א:א'). Some of the Meforshim are troubled by the Torah's use here of both the word אמר (talk) and the word ואמרת (tell) which each seems to convey the same instruction; there thus seems to be a redundancy in this Posuk (שם) because Moshe appears to be told twice to convey a message to the Kohanim. Ibn Ezra (שם בד"ה אמר) suggests that the first term, אמר (talk), actually expresses the imperative to instruct the Kohanim concerning the Mitzvos which were already presented in previous Parshiyos, which they are required to fulfill like all other members of Bnai Yisrael, but which they must be particularly careful about because the Torah is in their hands, meaning that they are responsible, as leaders and teachers, to preserve the words of the Torah. He then explains (שם בד"ה ואמרת) that the second term, ואמרת (tell), expresses the imperative to instruct the Kohanim concerning the Mitzvos to be presented in this Parsha which apply specifically to them. A similar suggestion is offered by Seforno (שם בד"ה ויאמר ובד"ה ואמרת), as well as by Rabbeinu Bechaya (שם בד"ה אמור), who then also quotes the idea from the Midrash Tanchuma on this Parsha (אות ג') that the first term imparts the instruction to tell the Kohanim that in certain instances they may, and in fact must, become טמא, namely, in order to deal with a מת מצוה, a dead person who has nobody to tend to his burial, while the second term imparts the instruction to tell the Kohanim that in general, they may not become טמא for any other reason. The Ohr HaChaim (שם בד"ה ואמרת) suggests that the first term presents the commandment to talk to the Kohanim concerning their own obligation to avoid becoming טמא, while the second term presents the commandment to explain that other members of Bnai Yisrael are not permitted to cause a Kohein to become טמא. The Ramban (שם בד"ה אמר) states simply that the double language is used here, as it is elsewhere, for the purpose of emphasis, which he writes is necessary when the Mitzvah in question is an important one, as this one apparently is, or when it involves an activity which goes against people's normal habits.
Rashi, however, in his commentary on this Posuk (שם בד"ה אמור), chooses to present only the understanding of these terms אמר and ואמרת as interpreted by the Gemara in Yevamos (דף קי"ד.), which states that the intent of the Posuk is להזהיר גדולים על הקטנים, to warn the older people concerning the younger people, meaning that the older Kohanim, those over the age of Bar Mitzvah, must take responsibility for the young ones, those under the age of Bar Mitzvah, and see to it that they observe all these laws properly as well. According to the Sifsei Chachomim (שם אות א'), the double language of the Posuk (שם) thus indicates that Moshe must actually make two separate, distinct statements to the Kohanim: one to instruct them about their obligations concerning themselves, namely, that they must observe the laws presented here, and one to instruct them about their obligations concerning the younger Kohanim, who are under the age of Bar Mitzvah, namely, that they must make sure that the younger Kohanim are aware of these laws as well. This also appears to be the understanding presented by Rashi in his commentary on that Gemara in Yevamos (שם בד"ה אמור). Rabbeinu Eliyahu Mizrachi, in his commentary to Rashi on the Torah (שם), explains this idea as emerging from the Posuk (שם) in a slightly different manner; he suggests that the two statements implied by the Posuk (שם) are not both to be made by Moshe. Rather, the first statement, commanded by the word אמר, is to be made by Moshe and directed to the Kohanim, while the other statement, hinted at by the extra word ואמרת, is to be made by the Kohanim and directed to their children; it is thus as if Moshe was told to tell the Kohanim not only what they themselves must do, but also what they must tell their children to do. In any case, it is clear that this Posuk in our Parsha (שם) implies the existence of a Mitzvah to teach and train children who are under the age of Bar Mitzvah about certain laws. The aforementioned Ramban (שם), who also cites this derivation, as do many of the Meforshim referred to above, writes that we learn from here, as well as from other examples presented by the Gemara in Yevamos (שם) which indicate the same idea, that one may not help a child under the age of Bar Mitzvah to violate any prohibition in the Torah. This is certainly the implication of the Gemara in Yevamos (שם), though the emphasis there seems to be that the adult may not actively or physically cause a child to commit an Aveirah, but, as the Ramban (שם) notes, if the child commits the Aveirah on his own, we are not commanded to stop him from doing so; this issue, however, is subject to some discussion, as will be presented below.
The Rambam (פרק י"ז מהל' מאכלות אסורות הלכה כ"ז) rules that it is prohibited to give a child forbidden food to eat, adding that this applies even to food which is forbidden MideRabbanan, and that it is also prohibited to make a child become accustomed to violating the laws of Shabbos or Yom Tov, even if the laws are only MideRabbanan. The Terumas HaDeshen (ספר תרומת הדשן פסקים וכתבים סימן ס"ב) explains that the reason an adult may not actively cause a child to commit an Aveirah is in order that the child will not become accustomed to doing such acts and then continue to do these things after reaching the age of Bar Mitzvah; this is similar to a concern raised by the Gemara in Shabbos (דף קל"ט. ועיין שם בתוד"ה וליתן) and in Eruvin (ועיין שם בתוד"ה דלמא דף מ:) and in Niddah (דף ס"ז:) that certain things, while perhaps technically permissible, can not be done because they may confuse children into thinking that they may engage in those kinds of activities even as adults, when they would be forbidden. The Chasam Sofer, however (שו"ת חתם סופר חלק אורח חיים סימן פ"ג), in discussing whether a retarded child may be placed in a situation, for his own good, where he will have to violate certain Aveiros, questions the position of the Terumas HaDeshen (שם) and indicates that it should be forbidden to cause a child to commit an Aveirah even if this will not cause him to learn something that will mislead him as an adult; Rav Chaim Ozer Grodzenski (שו"ת אחיעזר חלק ג' סימן פ"א אות א'-ו'), though, defends the view of the Terumas HaDeshen (שם) based on several sources. It should be noted that the simple reading of the aforementioned Gemara in Yevamos (שם) indicates that this prohibition to actively cause a child to commit an Aveirah is MideOraisa, as seems to be understood by the Rambam (שם) and others, and as is implied, at least with regards to the laws of becoming טמא, by the Sifra on this Parsha (פרשה א' הלכה א'), albeit based on a different phrase in the Posuk in our Parsha (שם). It seems that the Tur (או"ח סימן שמ"ג), however, as explained by the Beis Yosef (שם בסוף ד"ה והוי), holds that this prohibition is only MideRabbanan, an idea mentioned as well by the Pri Megadim (פתיחה כוללת חלק ב' אות ט'). The Pri Megadim elsewhere (או"ח שם במשבצות זהב ס"ק א') elaborates on this dispute, while the Noda BeYehudah (שו"ת נודע ביהודה מהדורא תנינא חלק או"ח סימן א') suggests that the prohibition may in fact be MideOraisa, even according to the explanation of the Beis Yosef on the Tur (שם), but it is not necessary to base it on Pesukim in the Torah because it is dictated by logic (מצד הסברא); this suggestion is actually advanced in the Moshav Zekeinim (a collection of comments on the Torah from the Baalei HaTosafos) on the Posuk in our Parsha (שם בד"ה ד"א אמור-האחרון).
As mentioned above, the Rambam (שם) writes that it is prohibited to cause a child to violate an Aveirah even if it is forbidden only MideRabbanan, and the same idea is presented by Tosafos in Rosh HaShanah (דף ל"ג. בד"ה תניא), based on a previous Gemara in Yevamos (שם), and is also implied by the Mordechai in Shabbos (סימן שס"ט, דף ע"ד: בדפי הרי"ף); the Shulchan Aruch (או"ח שם סעיף א') thus rules accordingly. If, however, a particular activity is not itself an Aveirah, but it will prevent a child from fulfilling a Mitzvas Asei, an adult may encourage or help a child to do that activity, as presented by the Hagahos Maimoniyos on the Rambam (פרק כ"ט מהל' שבת אות מ'-הראשון) and by the Terumas HaDeshen (שו"ת תרומת הדשן סימן צ"ד, קכ"ה), as well as by the Beis Yosef on the Tur (או"ח סימן רס"ט בד"ה והמקדש) and by the Derishah (שם אות ג') who discuss the custom to give the wine from the Kiddush recited in Shul on Friday night specifically to a child, even though the child will not be eating in the Shul. The Magen Avraham (שם ס"ק א') seems hesitant to accept this position, but he writes elsewhere (שם סימן תע"א ס"ק ז') quite clearly that one may actively help a child do something even if it will prevent his fulfillment of a Mitzvas Asei. The Sdei Chemed (אסיפת דינים, מערכת חמץ ומצה סימן ו' סוף אות ו') quotes an authority who allows permitting a child to do an activity which is prohibited only as a result of a Minhag or a special stringency (חומרא בעלמא); the Pri Megadim (שם סימן ש"א באשל אברהם ס"ק ל"ב) suggests that it may also be permissible to help a child do something which is prohibited only as part of a גזירה, that is, a decree designed to forbid an activity because it may lead to something else which is truly forbidden, although later (שם ס"ק ל"ה) he appears to be in doubt about this.
It must be pointed out that according to the Rashba, both in Shabbos (חדושי הרשב"א לדף קכ"א. שם בד"ה שמעת) and in Yevamos (שם בדף קי"ד. שם בד"ה רבי), it is in fact permissible to encourage a child to do an activity which is prohibited only MideRabbanan; the Ran in Yoma (דף א. בדפי הרי"ף בסוף ד"ה יום) appears to agree to this, and the Nimukei Yosef in Yevamos (דף מ"ב. בדפי הרי"ף בד"ה ומדאמרינן) quotes this idea as well. It is clear from these sources, though, that those authorities permit such an activity to be done only if it's for the benefit of the child himself, but it is improper to have a child engage in such an activity strictly for the benefit of an adult, as stated clearly by the Ran in Shabbos (דף מ"ה: בדפי הרי"ף בסוף ד"ה אבל); Rabbi Akiva Eiger (שו"ת רבי עקיבא איגר קמא, פסקים סימן ט"ו) thus rules that it is forbidden, for example, even according to these authorities, to give Seforim to a child to carry to Shul for him on Shabbos (where there is no Eiruv) even though this may be forbidden only MideRabbanan. It is noteworthy, though, that the Rashba himself in a Teshuvah (שו"ת הרשב"א חלק א' סימן צ"ב) indicates that he presented his ruling on this matter as a definition of the absolute Halacha, but he did not intend it to be followed on a practical level (למעשה); he does, however, defend this position (שם), which is certainly widely attributed to him by later authorities, even though the Beis Yosef (שם סימן שמ"ג בד"ה ולענין) quotes this Teshuvah from the Rashba (שם) as representing his true viewpoint. It should be stressed, moreover, that the Ritva in Yevamos (חדושי הריטב"א לדף קי"ד. שם בד"ה ומי') writes clearly, as do others, that under no circumstances may one actively have a child violate any prohibition, even if it is only MideRabbanan; the Shaar HaMelech on the Rambam (פרק א' מהל' שביתת עשור הלכה ג') asserts that this is the view of the Rambam cited above (הל' מאכלות אסורות שם), and the Mishnah Berurah, in his Biur Halacha (או"ח שם בד"ה מד"ס), asserts that the Shulchan Aruch )שם( also prohibits an adult from having a child violate even something which is MideRabbanan, even if it's for the child himself.
The Gemara in Rosh HaShanah (דף ל"ג.) indicates that one may work with a child on blowing the Shofar on Rosh HaShanah even if it is Shabbos; the Rambam (פרק ב' מהל' שופר הלכה ז' ועיין בכסף משנה שם) rules accordingly, as does the Tur (או"ח סימן תקפ"ח), although there is some discussion about the precise age of the child in question, as elaborated upon by the Beis Yosef (שם בד"ה ואף) and the Bach (שם בד"ה ואע"פ), among others. Rashi (שם בד"ה תינוקות) writes that this allowed because it is necessary to teach the child about the Mitzvah, implying that it is permissible to have a child violate something if it is for the purpose of training him to do a Mitzvah. Rabbi Akiva Eiger (חדושי רבי עקיבא איגר מערכה ח' בד"ה והנה המג"א) writes that such an action is permissible only if the act being committed in order to teach the child the Mitzvah is forbidden only MideRabbanan, but one cannot help the child do something which is prohibited MideOraisa even for the sake of teaching him a Mitzvah; this also seems to be the position of the Ran in Nedarim (דף ל"ו. בד"ה אמר רבי זירא ודף ל"ז. בד"ה בקטן) and of the Rashba there (חדושי הרשב"א לדף ל"ז. שם בד"ה והכי), as presented by the Chasam Sofer there (חדושי חתם סופר לדף ל"ו. שם בד"ה שה), who asserts that most authorities hold that one cannot have a child violate a prohibition which is from the Torah even for the sake of a Mitzvah. According to Tosafos in Pesachim (דף פ"ח. בד"ה שה), however, one may have a child violate even a prohibition from the Torah in order to train him to fulfill a Mitzvah; the Magen Avraham (שם סימן תמ"ג ס"ק ג') accepts this view, and cites others who do as well. The Chasam Sofer (שו"ת חת"ס חלק ו' ליקוטי שו"ת סימן י"ג) suggests that this may be the basis for a difficult position taken by the Taz (שם סימן שמ"ו ס"ק ו') who allows having a child carry keys (where there is no Eiruv) on Shabbos, because this will enable the child to go to Shul and daven, which of course is a Mitzvah, but the Chasam Sofer himself (שם) strongly disagrees with this leniency. The Shulchan Aruch HaRav (שם סימן שמ"ג סעיף ח') rules without making any distinctions that for the sake of training a child to do a Mitzvah, one may have the child violate a prohibition, although he stresses that the Mitzvah must indeed be something which the child needs to learn himself, but for the sake of a Mitzvah for other people, the child may not violate anything. Rav Chaim Ozer Grodzenski, in the Teshuvah cited above (שם אות ו'), brings other examples of cases where a child is to be encouraged to do something which is ordinarily forbidden in order to be trained to do a Mitzvah, and the Netziv, among others, commenting on a Gemara in Chagigah (ספר מרומי שדה לדף ב. שם בד"ה והנה), discusses this issue as well. It should be noted that under certain emergency conditions, it may be permissible to tell a child to violate something forbidden by the Torah if the alternative is that an adult will do it, as presented by the Ramo (שם סימן רנ"ט סעיף ז'), by the Shulchan Aruch (שם סימן רס"ו סעיף ה'), and by the Magen Avraham (שם סימן ש"ה ס"ק י"ב), among others.
As stated above, the Ramban, in his commentary on the Posuk in our Parsha שם(), writes that if a child is committing an Aveirah on his own, we are not required to stop him. This issue is actually the subject of a discussion in the aforementioned Gemara in Yevamos (שם) where different opinions are presented; the Rambam (פרק י"ב מהל' שבת הלכה ז', ובהל' מאכלות אסורות שם, ובפרק ג' מהל' אבל הלכה י"ב) and the Shulchan Aruch (או"ח סימן שמ"ג סעיף א') rule that people indeed need not stop a child from committing an Aveirah if he is doing it on his own. According to Tosafos in Shabbos (דף קכ"א. בד"ה שמע), however, this is true only concerning a very young child who is not of the age to be educated (לא הגיע לחינוך), but if the child is older, then he must indeed be stopped from committing an Aveirah; the aforementioned Rashba in Yevamos (שם) mentions this idea as well, as do others, and it is quoted in the Shulchan Aruch by the Ramo (שם). The Chayei Adam (שם כלל ס"ו סעיף ג') accepts this position regarding matters which are MideOraisa, and the Mishnah Berurah (שם ס"ק ז') concurs. The Rashba himself )שם(, though, appears to hold that it does not matter how old the child is, and the Tosafos Yeshanim in Yoma (דף פ"ב. בד"ה בן) quotes from Rabbi Eliezer of Metz that there is no general obligation to prevent any child from committing an Aveirah; this idea is also found in Tosafos in Nazir (דף כ"ח: בד"ה בנו) and elsewhere. These sources do, however, indicate that although it is not the responsibility of the broader public to prevent any child from committing an Aveirah, it is incumbent on the child's father to do so; the Rambam (הל' מאכלות אסורות שם הלכה כ"ח, ובהל' אבל שם) rules accordingly, and this view is also found in the Shulchan Aruch (שם) and in the Ramo (שם). The Magen Avraham (שם ס"ק ב') notes that a father is obligated to prevent his child from violating even a prohibition that is MideRabbanan, and then adds that the child must have some minimal level of understanding in order for this prohibition to be in effect. The Mishnah Berurah (שם ס"ק ג') discusses several of the important issues related to this topic, and then states that although when it comes to training a child to fulfill Mitzvos Asei, the age when he must be so trained depends upon the nature of the Mitzvah and of the child, when it comes to stopping him from committing an Aveirah, this must be done as soon as the child can understand being told not to do something.