You can log in to the router and apply Access List (filters users by the MAC ID of the user's wireless adapter). Include your own wireless adapter but not the kid's. Simple enough to disable it later to re-admit user.
How about scheduling an internet connection? Most wifi routers have this option. You can schedule what time there will be internet connection. So you can limit internet within day time and then turn it off come...say 10 in the evening then turn it on again by 8am automatically.