Best kids TVShow based game?

My nephews want a video game for xmas, their mom wants it to be co-op, they have a Wii. Almost all of my games are on 360, and games on the Wii play differently. So thought I would ask for help here.
 
They really like Bakugan, Pokemon, and Ben 10. I don't have experience with the shows or the games for any of those. I would like opinions on what would be the best game based on one of those 3, for a 6 and 9 year old?

Viva Pinata... HANDS DOWN! 
 I didn't even know it was a show. All I know is that the game is awesome, and there aren't many titles quite like it.  

-finishes reading post- 
 
EXCCCEPT its not on the Wii. Well, poop. Was going to say the new Pokemon adventure game that come out on Wii, but that's not Co-Op either. I'm not familiar with Bakugan or Ben 10, so I can't help you there :( Sorry.

You've never played pokemon......
(pick a pokemon game, trust me)

" You've never played pokemon...... (pick a pokemon game, trust me) "

don't get them TV show games, i think you should get them some mario games. every kid in the 90's crew up with mario.

" @ShadowofIntent said:

" You've never played pokemon...... (pick a pokemon game, trust me) "

"

 Don't think there is any good coop game versions for those shows, but Pokemon have a great singelplayer (best if English is there native language).
 
also: Mario kart, new super Mario bros Wii and super smash brothers brawl      

" You've never played pokemon...... (pick a pokemon game, trust me) "

Pokemon if it has to be based on a tv show, but get them Super Mario Galaxy 1 or 2 if ti doesn't.

We talked about Mario, the problem is Mario NEVER goes on sale. The Ben 10 and Bakugan seem to be what is on sale right now. 
 
Only reason we thought about one of those three is that the boys talk about them all the time and it is currently the "in" thing at their school. Not that I am big on following the crowd, but when 9 times out of 10 you see someone certain names pop up it is rather obvious they would appreciate something in that category.