Gemara

25b (Mishnah): There were upper stories in Beis Avtinas and Beis ha'Nitzutz. Rovim (young Kohanim) would guard there;

Pirchei (young) Kohanim would sleep on the floor.

26b - Question: Why are they called Rovim above, and here Pirchei Kohanim?

Answer: "Rovim" are too young to do Avodah. "Pirchei Kehunah" are old enough to do Avodah.

Sukah 42a (Beraisa): If a minor knows to guard his hands, we rely on him to permit eating Taharos;

If he knows how to slaughter, one may eat from his Shechitah.

(Rav Huna): This is only if an adult supervises.

Pesachim 4a (Beraisa): Everyone is believed regarding Bedikas Chametz, even women and children.

Suggestion: They are believed (that the house was checked) because there is a Chazakah that the owner checked it. (Everyone is reliable for Bedikas Chametz).

4b - Rebuttal: No, the Chazakah is that it was not checked. We rely on what they say.

Defense (of Answer #1): There is a Chazakah that the owner checked it. The case is, this house was Muchzak to be unchecked, and a woman or child said 'I checked it';

Bedikas Chametz is only mid'Rabanan. Therefore, Chachamim rely on them for a mid'Rabanan law.

Avodah Zarah 38a (Beraisa): A Yisrael may leave meat on coals and allow a Nochri to turn it over until he returns from the Beis ha'Keneses or Beis Medrash.

Rishonim

Rambam (Hilchos Beis ha'Bechirah 8:5): In Beis Avtinas and Beis ha'Nitzutz, Rovim would guard.

Mishneh l'Melech: The Gemara says that "Rovim" did not reach (the age of) Avodah. The Mefaresh and Rambam say that they were minors. This is astounding. How can the Mitzvah of guarding the Mikdash be fulfilled through minors who are not commanded? Do not say that Leviyim guarded. Verses teach that (also) Kohanim must guard! Perhaps in Ba'al ha'Mokad, the guards were not minors. There it mentions only Pirchei Kehunah, but not Rovim. This is a poor answer. Verses teach that Kohanim must guard in three places. If so, they must be Bnei Mitzvah. Rather, they did not reach the age of Avodah, i.e. 20. The Rambam (Hilchos Klei ha'Mikdash 5:15) says that an Ish (Bar Mitzvah - PF) is Kosher for Avodah, but the other Kohanim would not let him serve until 20. The Rovim who guarded were Bar Mitzvah, but less than 20. Semag says that they were youths who did not reach Chinuch Avodah. He did not clarify what this is. Piskei Tosfos (Tamid 6) says that below 20, Kohanim are not Kosher for Avodah. I say that they discuss the Rovim of our Mishnah.

Rosh (Mo'ed Katan 3:32): One witness is believed to permit. We learn (Kesuvos 72a) from "v'Safrah Lah" - a Zavah counts by herself (seven days without blood, after which she can immerse and be permitted to her husband). We are not stringent due to one witness.

Beis Yosef (YD 127 DH Kasav Rivash): R. Yerucham brings the Rosh.

Rivash (245): A case occurred in which a boy six years old said that he used a Kli (with residue from what a Nochri drank) to take wine from a barrel of wine. He is not believed to forbid the wine, which had Chezkas Heter. One witness is believed about Isurim, but a minor is not believed, and all the more so for an Isur mid'Rabanan. He is believed about mid'Rabanan laws to be lenient, but not to be stringent. If a witness says that the wine is forbidden and the owner contradicts him, R. Tam permits and others forbid, but all agree that a minor is not believed. If a minor knows to guard his hands, we rely on him to permit eating Taharos. This refers to what is b'Yado (within his power). What is not b'Yado, he is not believed to change its Chazakah, to permit or forbid.

Mordechai (Avodah Zarah 830): The Yerushalmi explains that one may leave meat with a Nochri to turn it over, i.e. when he also left a child to guard it, so there is no concern lest the Nochri switch the meat.

Tosfos (Pesachim 4b DH Hemnuhu): Techumim is an Isur mid'Rabanan. We believe women and slaves for it, but not minors. Only when the minor matures, he is believed to say '(when I was a minor,) we used to go until here on Shabbos' (Kesuvos 28a). This is because it is not b'Yado at all, unlike Bedikas Chametz.

Rashba (1:26, brought in Beis Yosef OC 460 DH Ein): Matzah must be guarded, therefore one is not Yotzei with Matzah kneaded by a minor. Supervision does not help for Shechitah, according to Chachamim who require intent. Tosfos (Chulin 12b DH Man) says that others see, but do not teach him and admonish him to intend, so it is unlike 'Omed Al Gabav' that helps for a Get. With supervision, a child, who has a little Da'as, can knead for Matzah, and we are Yotzei with it. R. Yonah says that for anything that can be done through a Shali'ach, such as Shechitah or writing a Get, intent of a supervisor (David) helps like intent of the one doing it (Levi). It is as if Levi is David's Shali'ach. Chalitzah cannot be done through a Shali'ach, therefore a minor cannot do Chalitzah, even with supervision. If so, a minor can knead l'Shem Matzas Mitzvah. However, it seems that supervision helps only if there is an indication that he does Lishmah, e.g. writing a Get with the proper names. There is no indication in Shechitah or kneading.

Poskim

Rema (YD 127:3): A minor is not considered a witness who is believed about Isurim.

Shach (31): The Rivash connotes that the owner is silent because he does not know. If he were silent because he admits, the minor would be believed. The Rivash holds like the Rashba, that one witness is believed even if the owner does not know. The Rema holds that one witness is not believed even in such a case. He teaches that a minor is not a witness at all, i.e. even if the minor says 'you know that there are Raglayim l'Davar (supporting evidence)', or if one witness contradicts him and the owner is not there, or something that can be verified, and he says 'I will show you a Nochri pouring your wine', or a matter without any Chazakah of Isur or Heter, or when we know that one of two pieces is Heter and the other is Isur. In such cases, one witness is believed even without the owner's silence, which is like admission. A minor is not believed. However, if it was b'Yado, he is believed to change its Chazakah, to permit or forbid. The Rivash connotes like this. See OC 437:4.

Gra (29): If a minor knows how to slaughter, one may eat from his Shechitah only if an adult supervised. The Ritva (Sukah 42b) explains that the minor is not believed to say that he slaughtered properly. The Rivash says that we rely on a minor for Taharos because it is b'Yado. The Ritva said that a minor is not believed only for matters like Shechitah, in which there was Chezkas Isur. (A living animal is forbidden; we must know that there was Kosher Shechitah in order to permit it.) He is believed to guard (preserve the Chazakah). From here we learn to things without Chezkas Isur (Sa'if 4).

Rema (ibid.): In any case, if a minor is sharp and expert in a matter, and there is Raglayim l'Davar for his words, we are stringent if he testifies about an Isur. If he testifies to be lenient about an Isur mid'Rabanan, and there is no Chezkas Isur, e.g. Bedikas Chametz, he is believed. Chachamim believed him about mid'Rabanan matters. If there was Chezkas Isur, he is not believed at all.

Gra (32, Likut): For what it is b'Yado to fix, women and minors are believed even about Torah laws, such as Shechitah and Taharos, unless it is a big exertion or there is a Safek or reasons to be lenient. For mid'Rabanan laws, they are believed even for a big exertion or when there is a Safek or reasons to be lenient. If it is not b'Yado, a minor is not believed even for mid'Rabanan laws.

Shulchan Aruch (4): If there is no Chezkas Isur about a matter, just concern lest a Nochri switch or touch it, some say that we rely on a minor who reached the age of Da'as (understanding) to guard.

Beis Yosef (DH Kasuv): This is from Orchos Chayim says in the name of the Ritva. He adds that if there is Chezkas Isur, a minor is not believed to say that it was permitted. This is why an adult must supervise his Shechitah.

Shulchan Aruch (OC 437:4): If a house was Muchzak to be unchecked, and a (woman or) minor said 'I checked it', he is believed, if he has Da'as to check.

Magen Avraham (8): In YD 194:14, the Shulchan Aruch rules that a minor is not believed about Tevilas Kelim. The Rema is difficult. In YD 127:3 he says that Chametz is not considered Chezkas Isur, therefore a minor is believed. Chametz is Chezkas Isur, like the Rivash says! Rather, he is believed because it is b'Yado. The Beis Yosef (YD 120, citing Terumas ha'Deshen 46) connotes that according to the opinion that Tevilas Kelim is mid'Rabanan, a minor is believed about it.

Mishnah Berurah (17): This is even if there used to be Vadai Chametz there.

Kaf ha'Chayim (32): Chak Yakov (11) says that if there used to be Vadai Chametz there, they are not believed. The Pri Chodosh, Eliyahu Rabah and Chukas ha'Pesach say that they are believed, because Bedikah is only mid'Rabanan.

Mishnah Berurah (18): B'Di'eved, if he said that he checked, one need not check again. However, l'Chatchilah one should not command a woman or child to check, for it is a great exertion, and perhaps they will be lazy.

Kaf ha'Chayim (33): The Bach says that the Tur and Rambam hold that l'Chatchilah we rely on them. The Rosh brings unlike this from the Yerushalmi. The Magen Avraham and Chak Yakov say that one may not rely on them.

Mishnah Berurah (19): 'Da'as' is Chinuch.

Shulchan Aruch (460:1): We do not knead Matzos for the Mitzvah through a minor.