There are several programs available for restricting how many hours children spend at the computer and how they can use the Internet.
They are effective to varying degrees. You may want to check www.cnet.com for the latest reviews of some of these programs.
If you want to control the amount of time the children are on the computer, be it to surf the Web or play Diablo for countless hours, try TooMuchPC from BlairSoft; You've Played Enough, a program written by an AOL subscriber; or Enuff from Akrontech.
NetNanny, CyberPatrol and CyberSitter also help keep kids away from inappropriate sites.
On the Web:
www.cybersitter.com/
www.cyberpatrol.com/
www.netnanny.com/homehome.asp
www.blairsoft.com/
http://members.aol.com/YPEmail/
www.akrontech.com/

If you happen to have Windows XP Professional, you can use your privileges as an administrator to limit the time users can access the computer.