Take the power cord
OR do what we do: my daughters (ages 6 and 9) have their own computer which is in the family room right beside the tv. The moniter is facing the sofa so that if I'm watching a movie etc I can see exactly what they are doing.
Their only internet connection is an external USB wireless connection. If I can't moniter their surfing, then I fucking yank the reciever and take it with me. They can still play all their games etc, but can't surf.
Overboard? Maybe...but in my opinion a 6 year old and a 9 year old have no business surfing the net unattended.