my opinion is:
for girls:
1. babydow(by owlient)
2. girlgogames(not really a virtual world)
3. howrse(for horse lovers(another owlient game))
4. go goddess
5. barbie
for boys:
1. runescape
3. warkill
4. fablell online
5. ea online
for mix:
1. Club Penguin
2. moshi monsters
3. panfu.com
4. ourworld
5. whyville
Chat with our AI personalities
teensecondlife.com
there is mmorpg
Meez too :)
I think IMVU could do it?
I only know 1; Gabbo.