Nidah 46b - Obligation on a child to do mitzvos

In Brachos 48a the gemara says that one who eats a kezayis can make birchas hamazon for others. Rashi understands that since the one saying birchas hamazon is obligated m'drabonon, he can be motzi others who are obligated m'doraysa. However, Rashi asks that we find that a child who reached the age of chinuch cannot make birchas hamazon for an adult, to which rashi answers that a child who reaches an age of chinuch is not even obligated m'drabonon, rather it is a rabbinic obligation on the father to train him to do mitzvos. Tosafos asks on rashi -דוחק לומר בקטן שהגיע לחינוך קרי אינו מחוייב מדרבנן. They seem to argue whether a child who reaches an age of chinuch is considered to be obligated m'rabonon, or is it merely an obligation on the father not on the child himself.

However, from our gemara (as explained by rashi) this isn't plausible. The gemara says that if a מופלא סמוך לאיש is m'doraysa in that his vows are binding, he should receive lashes for eating what he is makdish. If it is not d'oraysa, not only should there not be malkus for the child, there shouldn't even be a prohibition. The rationale of the gemara is that unless there is a gezeiras hakasuv telling us that a מופלא סמוך לאיש is responsible for his actions, it isn't possible to hold him accountable for his actions at all. Rashi explains that if the concept of מופלא סמוך לאיש is d'rabonon, all they would institute is that other shouldn't eat what this child was makdish. They wouldn't impose an issur on the child himself - דקטן לאו בר קבולי עליה תקנתא דרבנן הוא. Rashi holds that had the Torah imposed a specific issur on a child, we would have to accept it. But, if the Torah wouldn't impose an issur on the child, it would be impossible for the Rabbonon to do so. According to this approach it is impossible to say that a child is "obligated" in anything. Others may be required to guide him, depending on chinuch and whether we say beis din metzuvin l'hafrisho, but he himself has no obligation to refrain from issurim.

Regarding the issue of קטן אוכל נבילות , we pasken אין בית דין מצווין להפרישו, that beis din isn't required to stop him. There is a Toasfos in Shabbos that says that when a child reaches an age of chinuch we would then say that beis din is obligated to stop him. The Rashash asks that our gemara is clearly against this. Our gemara is speaking about a child who is one year prior to being a gadol, yet applies the concept of אין בית דין מצווין להפרישו. The Rashba also raises this question.