Commentary for Deuteronomy 31:13

ובניהם אשר לא ידעו, seeing that due to their tender age they are unable to formulate their questions if they have any.

ובניהם אשר לא ידעו, "and their children, who do not know, etc." This division is in line with mention of "and the children" in verse 12 as part of the מצוה of הקהל. Moses says that although he is aware that the children are too young to understand what they would hear at that assembly, they are not too young to learn to fear G'd, i.e. one must train them in יראת השם from an early age.

ובניהם אשר לא ידעו ישמעו ולמדו ליראה, “and their children who do not know of this will learn and be in awe, etc.;” when they will hear about it and ask their parents who will instruct them in all aspects of Judaism and Jewish history. The word טף, frequently used to describes babies, in this case speaks of children that are already older and that can be taught. In the previous verse the Torah wrote:ילמדו ויראו, ”they will learn and be in awe,” instead of “they will learn to be in awe.” Our sages do not understand the word טף here as referring to children of school-going age, but to infants, and they explain that the reasons that the mothers were to bring them along was to receive a reward for their efforts.