vista has that built in - Parental Supervision options or some such.. you have fine control over internet browsing as well
Most routers also allow you to limit certain IP and mac addresses on your network, you can set times and even limit certain protocols (FTP, ssl, http for example)
since many sites switch to a ssl mode (https) for logging in, blocking https effectively means they can look but cant log in to spend all day updating status on Facebook!
question: How can kids have a "Status" if they never "do" anything??? :P