Have you ever heard this phrase? I have, and it's always in situations where I don't like it.
For instance: "Don't expect your children to clean up around the house. Let kids be kids."
Or, "Don't encourage your children to care for baby dolls. They're babies themselves! Let kids be kids."
Or, "Don't give children any responsibility. Let kids be kids."
I hate this. The very nature of being a child is about learning to be an adult. Kids love to pretend to be grownups. They blossom when given responsibility. They enjoy tending dolls, or if they're lucky, real babies. They don't complain that their childhood is being taken away. They are proud. And when teenagers, especially, are given real work and real responsibility, they are happier and get a chance to prepare for adulthood. Teenagers don't say, "Let me be a kid." They say, "Let me be an adult."
Here's where I would like to see this phrase used instead:
"Don't advertise to children. Let kids be kids."
"Dress children in age-appropriate clothes. Let kids be kids."
"Give kids unstructured time every day. Let kids be kids."
"Don't encourage romance at a young age. Let kids be kids."
"Use clean language around children, and don't let them watch violent or inappropriate shows. Let kids be kids."
Those are some things I could get behind. But keeping kids in a box to shelter them from the good things grownups do, the things we would like them to do when they are older, is just keeping them from learning how to grow up. Let kids be kids, but don't stop them from trying to be the best of what adults are either. That's part of being a kid.