Like said above, the gamecube is ideal for the younger kids (8 and below) Cartoon style games, along with the Mario Brothers etc. and allows for 4 players.
Then comes the battle between PS2 vs Xbox (mainly for the age group 13+) Pros and Cons for both. I prefer PS2 games, but the Xbox has a bigger system with 4 players, which can be maximized to 32, via networking. You can play online with the Xbox if you have a lan, but you'll need to buy a subscription, Voice Communication is integerated aswell. Xbox probably is better for sports games, PS2 any other type.
Good luck.