DNS and censorship firewalls combined with modified browsers to protect children
A DNS could be provided to protect the children whereby the parents select the computer user account as a child's one and then the defatult DNS would automatically be altered to it this would have none of the porn websites and offensive websites on the internet; the selection of a child's account would also have the computer automatically routed through a firewall that not only provides moderation but possibly malware protection too (this could be a paid extra but should really be free).
As mentioned by Tyler Amos in an above suggestion certain modified browsers could be supplied too. My only variation to this would be that the browser would be supplied in the OS and would be switched to seamlessly upon the user account status, essentially an IE for adults and an IE for childeren this also means that the computer must have 3 rather than 2 types of account Administrator, Limited, and Child. added to this the browser would utilise the in built moderation feature he mentioned and it would automatically alert the DNS and firewall of the site for re evaluation, by a network administrator/team.