I liked Mario Party 4, but my friend that owns it says it can get tiresome. He's sixteen, though, so perhaps it will keep the interest of younger children longer. Mario Party 5 is coming out, though I'm not sure when. It could be a few months, so if you want it immediately it would have to be mario Party 4.
Super Smash Brothers Melee is a very, very good multiplayer game. Super Monkey Ball is, too, though I don't own it (the previously mentioned friend does). If you do get Monkey Ball, though, you should probably get SMB2, since it's already out.