In 31:13 Moshe is instructing the people regarding the mitzva of Hakhel. Every shemitta year when the people will come to the Temple during the Festival of Sukkot, the king is to read the whole Torah so they should hear and listen and learn to fear Hashem and keep His Torah. In passuk 13 Moshe then says "And the children who did no tknow will listen and learn to fear Hashem, etc.."
Why did the children not know? We educate our children all year round? We send them to school to learn Torah every day and describe the miracles of the exodus from Egypt and the desert? Why do they not know? Are we only to instruct them once every 7 years?
"The children who did not know" - sure we teach them. We send them to the best schools for them to learn Torah. We fulfill the mitzva of "And you should tell to your children" every Pesach seder. We teach them all the time. But they still do not know - it is not firsthand knowledge and cannot really be categorized as knowing it internally.
The next generation (and all subsequent generations) learns and hears and studies, but it is never going to be firsthand knowledge like that of those who experienced it, and therefore do not internalize it the same way.
By coming to a mass meeting once in 7 years with the whole nation to learn Torah together and review the events of the Torah that is the best and closest way to convert all that information they had learned into a sense of being together as one nation with a common goal of serving Hashem and developing this knowledge and sense of awareness.