What to do with a Child who plays SNG's?

Tell him that his bankroll has grown enough that he can now play a new game called the "college savings account" in which his money makes itself. After this give him his initial 100 back and keep him playing at a small level as you obviously have no qualms about him gambling in the first place. Perhaps this is where you want to instill some bankroll management skills that could actually become helpful later in life when he realizes you cant retire on pocket aces.