You'll need to talk to the Counselors. They usually let the "almost" 10 go in the 10-12 group on a trial basis and stay if it works out OK. I'm guessing that your 9 yo is used to hanging with the older group since he has a brother that age so I don't see a problem. Since he'll have a sibling in the older group, they may even expect that you'll want to move him.