Moshe informs the Jewish people of the Mitzvah of HAKHEL. He instructs them that after seven years, during the year of Shmitah, all the people were to be gathered together and have the Torah read before them. Although it is already in the eighth year and Shmitah has ended it is still called Shmitah because the produce of that year is still to be treated with the holiness of Shmitah.
Moshe is not satisfied with his words to gather “all the people” but actually enumerates who he has in mind. He says: HA’ANASHIM VEHANASHIM VEHATAF, “the men, the women, and the children…”(Deut. 31,12). The question many ask is that we can understand why the men and women were to be summoned but why the children.
The answer Rashi gives on the verse, based on the teaching of Chazal, is that the reason they are commanded to bring the children is to give reward for those who bring them. Someone asked, surely they had to bring them. Since the men and women came with whom were they to leave the children? Indeed it is true that they had no choice. However, as soon as they are commanded to bring them it becomes a Mitzvah and they are rewarded for fulfilling the Mitzvah.
There are many things that we would do even if it were not a Mitzvah, but since it is a Mitzvah, that is why we should do them and not because we think it is the correct thing to do.