[Kereth]

Personally, I'd recommend DiscworldMUD for this. The gameplay isn't really engaging, but the environment is loads of fun, and it seems to be decent and family-friendly, as far as I've seen. The only thing about it not meeting your specifications is that it can be pretty crowded sometimes. Most of the coded game seems to just be city terrain, so if you hang around the largest city, you'll be tripping over other players left and right, so not too much available in the way of isolating your class from other players.

-Kereth Midknight