What does “let kids be kids” mean in American English?

Let kids be kids is a common American English phrase that means “let children act like children.” Parents and teachers use it to push back against filling kids' days with too much: too much homework, too many lessons, not enough free time. It belongs to a family of “let X be X” sayings: boys will be boys, let girls be girls, kids be kids. They all argue that the natural way is the right way.

In this clip, a strict mom tells her kids to put down their books and go play. They want to study and learn piano. She says “no, no, kids be kids,” takes them to the park, and orders hot dogs.