Web browser/program for young kids?

I'm getting a new laptop (my current one hs some minor electrical issues that were starting to feel dangerous. I'm thinking of passing the old machine on to projectmothra. I'd like to give him 'net access, since he likes to IM bard_bloom and play webgame tie-ins to kids' movies. And though of course I'm not going to leave him unsupervised wth it for big chunks of time, he's already surprisingly good with google searches, and I think I'd be a bad parent if I didn't put some auto-filtering in place. Does anyone have experience with any of the content-restriction or opt-in kid's browsing programs?

Thanks.