Xbox or PS3? I had the same thing happening when my adult sim changed into an elder. When I let her access the create-a-sim mode through a mirror and changed some things (her hairstyle), she was fine again.
As a last ditch effort: how about using Karma to change your sim into a pet (and back again) or possibly a ghost (and then feed Ambrosia). Perhaps forcing the game to render your sim in a different way will fix it? I'm just not sure these powers will work on a childsim, though you could make him eat a birthdaycake first.