The simpilest way to do it without causing any problems it to
set up a task that occurs on every login.
First create a .bat file Use Notepad and name it ipconfig.bat and
cut and paste this in there
Save file type as All Files in the drop down menu.
Save this file in the \Winnt\System32 directory in Windows 2000
or \Windows\System32 in Windows XP
Go to Start, Settings, Control Panel, find Scheduled Tasks, open it,
hit Add Scheduled Tasks, browse to the ipconfig.bat file,
Select When I log on, type in the Administrator name and password. Click Next and Next again.
This will cause the machine to release it's IP address on
every logon and therefore you cannot get to the internet.
Now for you when you want to surf the internet you will have to
go to Start, Run, and type ipconfig /renew
This will give you an IP address to be able to surf the net.
If the kids are computer savvy and figure this out just send out another question and I will give you a trick to keep them scratching there heads for weeks trying to figure out whats wrong.