Suggestions for parental control software?

Got my nephew started on ubuntu in the last year and the little guy is already pretty good with python and using rudimentary bash commands. I want to install a software system on his computer that will ensure that he can't visit any porn sites.

Any suggestions for what I should use?

My first thought was to use the /etc/hosts file to resolve all but a few website IPs to 0.0.0.0, but that seems too restrictive.

Thanks!