Taking option one from orac and the percolation of computers into kids' rooms I've a suggestion for some brutally effective parental control of surftime.
I've just spotted the latest wireless cards that look identical to usb memory sticks. Assuming the rugrats' computers are usb equipped an extra tenner buys you a wireless card for their machine that is only on when you want it to be - i.e. a simple, and if needed, dramatic withdrawal of said link to the outside world.
If I disappear from the site without trace you can take it my little dears didn't like the idea
Regards
Rob