[quote name='luan87us']get a Wii you and your kids will enjoy that more[/quote]
no...Save your $250 and put it towards some good games for the 360. My five-year old and I play games on the 360 way more than the wii. She enjoys Puzzle Arcade (we haven't tried out the co-op mode yet but she enjoys playing that by herself). We play marvel ultimate alliance, that dorito dino game, and viva pinata party animals (I HATE that game).
I would pick up the 5 dollar Arcade disc from gamestop (It's green and comes with the arcade bundles). My daughter and wife love the games on there Uno, Pacman CE, Luxor, Feeding Frenzy and Boom Boom Rocket.
Beyond these, the lego games and music games that you've already mentioned, I'd look into Street Fighter II Turbo HD Remix, The Maw (Single player only, but my daughter enjoyed the demo and plays it over and over...I should buy it!), Tony Hawk/Skate games (I don't have skate so I don't know about co-op, but I know project 8 has it and it's fun), and Bomberman Live.
Honestly, I would also go ahead and get/let her play Halo. That's a blast to play co-op through the game and as long as she isn't playing online matches, I don't see a problem content wise. Maybe that's just me though.