My input on topic.
Your kids WILL get bored of the Wii, The only true games to come to Wii in my eyes, were Mario Galaxy and RE4, everything else has been PARTY GAME, PARTY GAME PARTY GAME, or just dull and/or badly implimented.
The 360 has a massive games catalogue with a lot of games for kids.
LOADS of sports and driving games. Wrestling games, etc.
Loads of movie crossovers - Open Season, Wall-E, Bolt, Monsters Vs Aliens
The Lego series are great, Star wars, Batman and Indiana Jones.
There's the Sega ultimate collection which has loads of old Sega games on it, Plenty of Sonic!
a lot of these are on PS3 too, But as said, its likely your kids friends will not have a ps3 because they are pricey, so the 360 would be best so he can play against his friends