A review for "facebook for kids" makes me think this is a bad idea: http://www.theregister.co.uk/2008/12/12/school_together_now/
Anyone can pretend to be a kid, and join the site.
I think it's good to protect kids a long as possible from this kind of exposure. Even on this site, I sometimes am concerned that members are not always moms or people who have good reason to be on the site. You cannot be too careful with your child. I do not think regular facebook is good/necessary for a child, either. Parents do not always know how to regulate usage, and I have seen some things I do not want to see from kids who are young cousins of mine (ages 11 and 8, and they advertise themselves as 18 and 16... their mom is clueless about this, and they are talking to boys, and I think are way too trusting of the online stuff, in my opinion).