+1 on that, Second best is removeing the power cable and bringing it into my bedroom ;)
First off what OS are we talking about, XP has some nice features and password ability.
Programs such as Netnanny and such are only as good at the user who sets it up, and also if you want to shut the Inet off totally. Load up a firewall program such as Zonealarm, put a password on it, and lock all ports in and out when you aren't using it, safe for the computer and the kids.