Gemara

113b: R. Yitzchak bar Bisna lost the keys to the Beis Medrash in a Reshus ha'Rabim on Shabbos. R. Pedas said that they should get minors to play there. If they find the keys, they will bring them. He holds that if a minor is eating forbidden food (or doing any other Isur), Beis Din need not stop him.

Question (Mishnah): If a minor comes to extinguish a fire, we tell him 'don't extinguish', since we are commanded that he rest on Shabbos.

Answer (R. Yochanan): It is forbidden when the child does so to please his father.

Support (Beraisa): If the son of a Chaver (one trustworthy about Ma'aser) often goes to his mother's father's house, we are not concerned lest they feed him Tevel. If the father finds food in his son's hand, he is not responsible.

Rejection (R. Yochanan): This is no support. We are more lenient about Demai (doubtfully tithed produce).

Inference: If it was definitely Tevel, he would need to Ma'aser it.

Contradiction: Above, R. Yochanan answered 'he acts on his father's volition' (but we need not stop minors who transgress for their own benefit)!

Answer: R. Yochanan is in doubt what is the Halachah. He rejects the proofs for both opinions.

Support (Beraisa): If the son of a Kohen Chaver (who is reliable regarding Taharah) often goes to the house of his mother's father's, an ignoramus Kohen, we are not concerned lest they feed him Tamei Terumah. If the father finds food in his hand, he is not responsible.

Rejection: This is no support. The Beraisa discusses Terumah mid'Rabanan.

Question (Beraisa): "Lo Tochlum (do not eat Sheratzim)" - we read this 'Lo Ta'achilum (do not feed them)', to warn adults about minors.

(Beraisa): "Every soul among you will not eat blood" warns adults about minors.

(Beraisa): It says "Emor" and "v'Omarta" (before the Lav for Kohanim to become Tamei), to warn adults about minors.

Suggestion: One must tell minors not to eat Isur or become Tamei!

Answer: No, the verses forbid giving them Isur to eat or making them Tamei. All three verses are needed;

Had the Torah taught only about Sheratzim, one might have thought that this is because the Isur applies to any amount, but adults may feed blood to minors, for the Shi'ur (to be liable) for blood is a Revi'is;

Had the Torah taught only regarding blood, one might have thought that this is because there is Kares for eating blood, but adults may feed Sheratzim to minors.

Had the Torah taught only these two, one might have thought that this is because the Isur applies to everyone, but since Tum'ah applies only to Kohanim, adults may make minors Tamei;

Had the Torah taught only regarding Tum'ah, one might have thought this applies only there, since the Torah gave extra laws to Kohanim, but adults may give to minors Isurim that apply to everyone.

Rishonim

Rif (Hilchos Tum'ah (after Menachos) 1a): "Emor... v'Omarta" warns adults about minors.

Nimukei Yosef (DH Tanya): This is extra to warn a father not to allow his son to become Tamei. In other Isurim the father is not commanded. We do not allow a child to extinguish; this is only when he does so for his father. Regarding Tum'ah the father is always commanded.

Rambam (Hilchos Ma'achalos Asuros 17:27): If a minor ate something forbidden or did Melachah on Shabbos, Beis Din is not commanded to separate him, for he does not have Da'as.

Kesef Mishneh: Even though R. Yochanan was unsure, since R. Pedas was sure and acted upon this we follow him, in spite of R. Yochanan's doubt.

Rambam (ibid.): This is if he acts for himself. One may not feed him Isur, even an Isur mid'Rabanan.

Rambam (28): Even though Beis Din is not commanded to stop a minor, his father must rebuke him and separate him in order to train him in Kedushah - "Chanoch la'Na'ar Al Pi Darko".

Magid Mishneh: If he father does not rebuke him, Beis Din is commanded to invigorate the father to rebuke his son to stop him.

Nimukei Yosef (42a DH Garsinan): The Rashba says that this is reasonable. Since we train him to do Mitzvos, surely we must prevent him from Isurim! However, regarding a Mufla Samuch l'Ish (a minor shortly before adulthood) the Gemara suggested that Beis Din is commanded, and rejected this (Nidah 46b) Perhaps Chinuch applies only to Mitzvos Aseh, but not to Lavin! The Ritva says that in any case we are not lenient to feed an Isur mid'Rabanan even to a minor below Chinuch.

Rambam (Hilchos Evel 3:12): Adults are commanded not to make minors Tamei. If a minor wants to Metamei himself, Beis Din is not commanded to stop him. His father must train him in Kedushah.

Source (Radvaz): The Sifri says that we are commanded about minors because they will come to be commanded.

Question (Tur YD 373): The Gemara expounded "Emor...v'Omarta" to warn adults about minors, i.e. to warn them not to become Tamei.

Answer (Beis Yosef OC 343 DH v'Da): The Gemara answered that it forbids adults to Metamei a minor, but they need not stop him from becoming Tamei!

Beis Yosef (DH v'Nir'eh through DH v'Af): In conclusion, there are three verses ha'Ba'im k'Echad (that teach the same principle, that adults are commanded about minors). All agree that we do not learn from three verses to elsewhere, but surely we may feed minors (or prompt them to do) other Isurim! Rather, we must say that in these three Isurim Beis Din must stop minors from transgressing. Even though the Gemara said that we could not learn from one of them to the others, the Tur holds that this is not really true. If it were, two verses would have sufficed. We could learn everything from Tum'ah and Neveilah. Blood is Chayavei Kerisos, it is more stringent than Neveilah. Also Sheratzim are more stringent than Neveilah, for one is liable for any amount. Alternatively, we could learn from Tum'ah and blood. Rather, all three verses were not really needed. We learn the Beraisos simply, to teach that adults must warn minors about these three Isurim. Also the Nimukei Yosef (Hilchos Tum'ah 1a) says that the verse commands about Tum'ah even when the child acts for himself.

Note: The Nimukei Yosef obligates the father to warn his child. He does not obligate Beis Din.

Rosh (Shabbos 1:1): Beis Din is commanded to stop a minor from eating Isur, all the more so we may not help an adult transgress!

Hagahos Ashri (Shabbos 9:2): We do not give a live (Tamei) locust to a minor, lest it die and he eat it. Even the opinion that Beis Din is not commanded to stop a minor forbids this.

Kaf ha'Chayim (OC 343:13): Nowadays people do not eat locusts, so one may give it to a child to play with.

Nimukei Yosef (42a DH Garsinan): The Halachah is that Beis Din is not commanded to stop a minor. The Gemara here and in Gitin (55a) asks as if this is known, and R. Pedas acted upon it in practice. No Amora disagrees. We answered every question against him. The case of extinguishing to please his father is when his father is there and indicates that he desires this. This is like telling him to extinguish; we do not feed Isur to a minor (or overtly cause him to transgress). The Rashba holds that for mid'Oraisa Isurim we need not stop minors, but we do not feed them. We are more lenient about mid'Rabanan Isurim and may feed them to minors. The Rambam does not allow feeding even Isurim mid'Rabanan.

Poskim

Shulchan Aruch (YD 373:1): Just like a Kohen is commanded not to become Tamei, adults are commanded about minors.

Rema: They are commanded not to overtly be Metamei them. OC 343 discusses whether or not we must separate them if they are becoming Tamei by themselves.

Taz (1): Adult Kohanim are commanded about minors. The Tur says that Beis Din need not separate hem. This implies that Beis Din may not be Metamei them.

Be'er Heitev (1): The Agudah says that one need not wake a minor sleeping in Ohel ha'Mes, but it is good to do so for Chinuch. This does not apply to an infant without Da'as who needs his mother.

Question (V'Shav ha'Kohen Sof 56, cited in Pischei Teshuvah 1): The Rambam says that Tum'as Kohanim for relatives is permitted due to Aveilus. A minor is exempt from Aveilus. Is he therefore forbidden to be Metamei for relatives? Or, perhaps Chinuch does not apply to something that will be permitted when he is an adult!

Beis Yosef (154 DH u'Mah she'Chosav Aval): One need not protest if a Nochris wet-nurse feeds Isur to a child, for Beis Din is not commanded about minors. One may not tell her to feed Isur to the child; the Rashba permits an Isur mid'Rabanan. The Rambam requires the father to stop the child.

Shulchan Aruch (343:1): If a minor is eating Neveilos, Beis Din is not commanded to stop him, but his father must rebuke him and separate him (Rema - from an Isur mid'Oraisa).

Shulchan Aruch (ibid.): One may not feed him even an Isur mid'Rabanan.

Gra (DH v'Afilu): We may not Me'arev with Tevel mid'Rabanan, for we may not feed it even to minors. The Rashba permits feeding an Isur mid'Rabanan to minors, but not consistently lest the child continue to do so after maturing.

Rema: Some say that this refers to a minor below the age of Chinuch, but once he reached Chinuch we must stop him. Some say that Chinuch applies only to the father, not to Beis Din.