I agree that it's a good idea in principle but I'd be happier if it were a closed board, that is, a board that could not be read by adults apart from global moderators.

General Chat is moderated. Children are still going to read the harsher topics. Child curiosity will see to that!
I don't see the problem with children posting if they want to. Perhaps it's the adults who feel threatened, even embarrassed?
Putting them on their own board is only going to draw attention to them. They (and their parents) might get a false sense of security.

Blimey, I sound like the prophetess of doom!

A bad idea on a few counts:

1. Adults would post.
2. Children want to have conversations with adults. It wouldn't stop them posting on an adult board.
3. Paedophiles this way....
4. Are you trying to say there's adult conversation on General Chat? I couldn't have been online that day!
5. Adults pretending to be children would post.