I don't think of it so much as not letting kids act like "kids"- I see it more like teaching kids how to act considerately of others around them. I was taught gently from an early age, and taught my daughters as well, about stuff like:
-- sitting patiently in a restaurant rather than running about from table to table.
-- not using a loud shouting "outside" voice when inside.
-- not interrupting people when they are talking (wait for them to finish a sentence and say excuse me first).
-- not insulting people because of how they look, etc.
-- not screaming in a tantrum fit when my mother didn't buy me something I wanted in a store.
-- not PUSHING ahead in front of old people when going through doors (i see this all the time), but holding the door for them instead and giving up seats on the bus for people who are not as strong as me.
-- eating with my mouth closed, and not talking with a mouth full of food.
-- saying please and thank you and excuse me.
I had no trouble learning these things from the time I was 4 or 5. They were repeatedly explained simply and kindly but firmly. I don't think learning to be kind and polite "ruined" my enjoyment of childhood or stifled me emotionally.

I had a pretty unrestricted childhood.