I'm opposite to you, the NDS belongs to me and I will borrow it to my son. So whether he can play depends on me and he must ask me before he can play. I'll bring it with me or put it away when I"m not home. So far so good for my son.
I bought one for him on his birthday (He is now 8 yrs old).He can only play on Friday night after finish homework and review, weekend after pratice instructment, and some reading. During meal, cannot play.