From my own experience with that age group, Zack & Wiki as well as Wii Music would be very bad decisions (especially if they're boys).

Mario Strikers Charged would be a good choice though.

From those suggestions boom blox,zack&wiki,de blob and mario strikers look like the best ones. Any more suggestions?

@Khuutra

No, I don't want to hear them play wii music for few days. I have my limits! Also I don't know what dragonball is.

@Roma

They are quite bright kids, I would actually prefer a game which can be challanging.

edit:

Is zack&wiki really that difficult? I don't know much about it, but how is it compared to old games like king's quest?

Get Mario Strikers Charged.

 This!

Sonic and mario at the olympics baby

@ OP: Wait - what? You... don't know what Dragon Ball... is? Man. I salute you! :p

Still, get Boom Blox. You won't regret it.

Also, I've got Zack & Wiki - don't get that. It's hard. I had quite a bit of difficulty with it and I'm 23. Also, while de Blob's fun, I don't think it's really a great game for 6-8 year olds. It doesn't have any multiplayer either.
Mario Strikers is cool, but might be a bit complicated for them too. Maybe not.

Boom Blox is a bit Wii Sports-ish. Easy and intuitive, pick-up and play. They'll like it. So will you. ;) Plus, you can play together.

 Exactly what I was thinking.

The games a blast to play. Very solid choice.

Boom Blox
Mario Strikers
Mario Sluggers
Mario Party 8
Mario & Sonic At the Olympics
Any of the Lego games
Any of the Rayman's Raving Rabbids, especially 2 or 3

Hate to say it but they'd probably like Nerf N-Strike. Aside from the gimmick it's not a bad on-rails shooter.

de blob has multiplayer

Ah, you're right Cougarman. What was I thinking? :p Thanks for pointing that out.