You bought a Wii.  Good show, man.  I approve!

Multiplayer for a 10 year old would be Mario Kart, Wii Sports Restort, NSMBWii, SSBB, to name a few.

For YOU, I would suggest Monster Hunter Tri, Goldeneye, and Black Ops. I have seen some very good sales for Monster Hunter Tri at Future Shop. It has been on sale for 39.99 including the classic controller.

Black Ops for Wii? Hahahaha

Switch that out with something good, like Super Mario Galaxy.

Oops! Forgot the poster has all 3 consoles. My bad.

But its the only one with pointer controls ;)

If you like puzzle games with potential for user-created content, Boom Blox Bash Party is a great chioce.  The game features fun multiplayer and some awesome creations online.  And I bet it will be much different then anything you have on the HD consoles.

Go with Brawl first though.

Congratulations!  It's a fun system. Others I like for fun party mulitplayer is Sport's Resport and The Ravin' Rabbids games.

My family likes to switch off and play a big round of Frisbie Golf on the holidays, and the sword fighting games is fun and exhausting. There is a 'epic' mode where you sword fight across bridges, beaches, etc.; like epic scenes from movies.

And the first 3 Rabbids Games are funny/sometimes disgusting or naughty party games.  It gives you a good variety in a single game (shootin', racin', dancin',balacin' and more.)

Zappykins

Smash Bros and Mario Kart are still the two best multiplayer games on the Wii

Both Super Mario Galaxies are good for some co-op, although 2nd player does lack for things to do, especially in the 1st one.

There's a fair few good shooters out now for the Wii, either any of the recent fps or pretty much any of the rail shooters are good co-op fun (hotd is a good choice if you dont take your gaming overly seriously, dead space or resident evil are better choices otherwise)

For multiplayer NSMBWii, Mario Kart and Brawl are a must

Appart from the already mentioned MKWii, SSBB, NSMBWii; and for the Wii series games like Wii Sport Resort or Wii Party. You should look into  Big Brain Academy multi; or the Mario & Sonic olympic games, which has some frantic mini games.

But one that get's rarely mention is the Trauma Center series, and it's one I always look forward too to play with someone (Trauma Team is taking too long...), both New Blood and Trauma Team have coop.

And lastly for music games, both Just Dance or We Cheer series' are nice to move along, but for cheap you can get Samba de Amigo which also does a pretty good job.