http://pikluk.com/
Found this site, basically it creates an "intranet" with a list of safe sites, and keeps your kids out of your file system and out of bad sites, and also gives them safe e-mail.
Made me think of the whole .xxx ordeal, I think this is a good idea for parents as an alternative to blocking, plus it keeps them from ruining your PC's OS by deleting files or installing viruses etc