I think you'll find that some of these RPG's may be a bit too complicated for a 9 year old. You'd want a game that is easy hack'n slash instead, which Diablo should be able to fit that bill.
Now I cannot remember if this game had Multiplayer LAN support or not, but you can play on the same computer as it supports 4 players. A 1998 game called Get Medieval, which is pretty much just a Gauntlet clone, but quite a good one too.
And I don't know if you want to count split-screen gaming as an option as well, because there are PC versions of Dynasty Warriors 4, 5 or 6, Warriors Orochi or Samurai Warriors 2.