My son just turned 10. He has been rafting since he was 5 though (Deso, Cataract, Lodore, Yampa, Main Payette and Snake).
I think 3-5k would be the safest level to take the kids on with good backup group of friends who can run safety and chase the swimmers. 2k was an easy level, but Skull sure gave us a run for our boat sizes ha!
I've never gone to Salt yet, still on my list to do.