I'm confused by your requirements however you could;
- Remove the keyboard and mouse
- Install netnanny type software
- Remove the mains plug
- Lock the room in which the computer is stored
- Set up your network router to block internbet usage (If supported)
- Remove the router
- Unplug the adsl connection
- Unplug the router
- Delete the child's account
- Use windows 7 which includes a child protection system
- Sell your computer