Mine seems to allow it. Can be done with one router.
Sounds like it'd be dependent on the router's firmware.
First, I can set it to block range of IP's from using a service, say http, that would probably be good enough to kill the kids 'eh?
But you could add others like ftp, etc. if ya really needed to,
Then I can set that Block to a schedule, like never, everyday, or choose specific days of the week.
Then ya set a start hour/min, end hour/min to block.
You'd just wanna be clever in your choice of IP's to block, making sure your own PC
gets the same IP address everytime it gets an IP from the router, outside the range
of what your blocking.
Mine's a Netgear WGR614 V7. An otherwise pretty crappy router mind you.
Find myself restarting it more often than I'd like.