As many of the other posters said, it's technically for both. Here, I'll prove an example for you with certain games:
-Rayman Raving Rabbids- A ton of (easy) mini games, with a cool storyline.
-Rampage: Total Distruction- Easy to pick up game for the little ones, with a long comprehensive storyline.
-Cars- Based on the movie, as well as it being an older game, kids will still enjoy the control scheme as well as the characters from the movie.
-Excite Truck- An easy, but challenging and fun game with many unlockables.
*-Madden- Cool way to handle passes and rushes, as well as catching those long passes.
* means it may be a bit challenging for younger audiances, but it's for all ages.
Legend of Zelda: Twilight Princess- A game that pretty much challenges your integrity and eyes with witty puzzles and cool dungeons.
Red Steel- A game in which has a multiplayer killing spree, as well as wielding swords and guns.
Call of Duty 3- The first War game for the Wii, in which it may not have multiplayer like other versions, but the control scheme makes up for it.
*Need For Speed: Carbon- Need For Speed is for the older audiance, Cars is for the younger.
Blitz: The League- A Football league with no rules, blood, drugs and sex...need I say more upon why it's not for the kids?
Mortal Kombat Armageddon- Ported to the Wii, going to be one of the first mature rated fighting games.