My daughter really, really liked viva pinata... except for one little thing... the pinatas eat each other. She hated that part and didn't want to play it because of that. One of her classmates also bought the game and returned it for the exact same reason. On any forum with Viva Pinata content where parents of smaller kids contribute, you will find that is the only real complaint in the game. Imagine if Animal Crossing characters ate each other... Rare/Microsoft made a huge miscalculation in that one aspect of the game...
MS really needs to look at what Nintendo does well for games that can truly appeal to kids/family and execute on that same level.
As another side note, my daughter is 9. She likes her wii and DS but she also likes (on the 360):
Assault Heroes
Outpost Kaloki X
Band of Bugs
Viva Pinata Party Animals
Earth Defense Force
Samurai Warriors 2
Lego Star Wars
I hate trolls.

Systems I currently own: 360, PS3, Wii, DS Lite (2)
Systems I've owned: PS2, PS1, Dreamcast, Saturn, 3DO, Genesis, Gamecube, N64, SNES, NES, GBA, GB, C64, Amiga, Atari 2600 and 5200, Sega Game Gear, Vectrex, Intellivision, Pong. Yes, Pong.