For kids' computers, consider using a third-party DNS service such as OpenDNS. Once you're registered, it's easy to tell it which categories to block. As pure DNS, it's fast, too.
I would concur with this too. It's saved a lot of my customers when their ISP's DNS servers died on them too. FWIW it's pretty inexcusable to have an ISP's DNS service die on you, but a lot of ISPs haven't got a clue how to architect robust DNS anyway (evidenced by the number of DNS servers on the same c-class subnet IP, for instance)