AA doesn't discriminate based on age - everyone is subject to the same rules. I don't see any reason why different rules should apply just because we're talking about a minor. As the guardian you can claim awards out of the child's account.
You can pool miles at this site, but it's a waste of money:
http://shareaamiles.points.com
It's better to either wait until there's enough miles in the kid's account to redeem an award or just keep earning the miles and give them to the kid when she's of age.