If you check the "note to kids" section in Gaia Online, there's a note in there saying that Gaia online is NOT for kids under 13.
I did a lot of research over these things myself for my stepson. (he's 11 now)
I played MapleStory with him a while back, don't remember the SSN being asked (maybe because I put Canada in the location). It's much more age appropriate than Gaiaonline. (it's good old 2D hack and slash fun) I'd rather he play that than Runescape or Ragnarok online.
Webkins is possibly the safest community you can ask for, but the membership only last one year after you activate. It's a lot of fun for younger kids though. My niece loves it and there's no worries about letting her play alone either because she can't type in anything.
There's also Disney's Toon Town - it's also perfectly safe and a lot of fun. However, it's the MOST EXPENSIVE option. It costs as much as an adult MMO game subscription.
Here's a link if you want to do all the reading I already did:
http://www.virtualworldsreview.com/info/categories.shtmland if you need other reviews, I find this extremely useful:
http://kidconfidence.com(It's like gamespot for parents who wants to buy games for their kids - they also do books and toys and I use it like a bible.)
P.S. You're very welcome.
P.P.S. Have you looked into TyGirlz?