I strongly disagree. Kids are voilent animals with no respect for other people. They must be shown that the real world has consequences since early ages. In general, to "let kids just be kids" means protecting them from real world consequences. In doing that, you are raising entitled pricks (no always, but often).

You should also show them that even when there are no real world consequences (for example the 9 year old kid who killed with cruelty 28 pets in a pet farm in brasil, and wont be charged because he is under 12 y.o.), there are moral reasons for being a humane and caring person.

So no, don't "let kids just be kids", raise them to be responsible and humane persons.