The Credibility of Minors – Eruvin 31b
תלמוד בבלי מסכת עירובין דף לא/ב
והאמר רב הונא קטן גובה את העירוב לא קשיא כאן בעירובי תחומין כאן בעירובי חצירות:
Rashi understands the Gemara as teaching us that although a minor is capable of collecting the bread that serves as an eruv chatzeiros, he is incapable of making the "acquisition" that is the basis of eruv techumin. Tosafos (d.h.
Tosafos raise the question that, if so, why are minors believed when they say a house has been checked and is chametz-free (Pesachim 4b)? Tosafos respond that it is only concerning the house in which the minor himself lives that the Rabbis credited his assertion that the house was checked, as the minor [assuming he is old enough to understand the issue of chametz – i.e., "he'gi'ah l'chinuch] feels that responsibility incumbent upon himself. The minor would not be believed, however, concerning a house other than his own — even if he was specifically delegated the responsibility of checking the other house.
In terms of practical Halachah, however, the Mishnah Berurah (Orach Chaim 537:16) rules that a minor is even believed when he tells us that a house other than his own home was checked (see Shaar HaTziyun ad loc. §19). This is because the primary reason that he is held to be credible is because the entire checking procedure is only a "back-up" to the primary procedure of Bittul — the negation of the chametz. However, accordingly, the minor is only believed in conjunction with Bittul. Hence, if it is already so late on Erev Pesach that Bittul is no longer effective, the minor is not believed either, and an adult must check the premises.