I don't frequent this section of the board, so take it easy on me!
My nine-year old son likes to play Adventure Quest at battleon.com and I am looking for something in that vein for him as a stand alone computer game. I am not so keen on him being connected to the internet and gaming with strangers and would rather he had an age appropriate game he could play on the computer.
They seemed to like an RPG called Fate, which seems sort of in line with the sort of game you're looking for. Unfortunately, there aren't many kid-friendly RPG's for the PC, maybe the ports of the Final Fantasy games? This Freedom Force game looks kind of fun. http://www.mobygames.com/game/windows/freedom-force
Just in case you don't know, Adventure Quest IS designed for kids to play. While it is an online game, it is not one in which you interact with other players. The game consists entirely of you fighting computer controlled enemies, gaining higher levels/stats, and buying new equipment and abilities. The only way its being online is utilized are the very frequent equipment, ability, and enemy updates. There is no interaction with other players in the gameplay itself or via a chat window while one is playing. The only way to interact with other players is to if you happen to go to the game's forums.
HBK: Youíre flat broke arenít you? MJ: Please let me wrestle on RAW! HBK: Fine, but I ainít weariní no fruity tassels. MJ: Are these your mirror pants? HBK: Give me those!