Kinect requires a good deal of room.
Wii is a good call. Lots of family-friendly titles there. Arguably the best, and cheapest, choice to keep the kids happy.
At that age, there’s probably little to choose from between the Xbox and the PS3. Most titles are available for both platforms and are as close to identical as makes no odds.
Do you know other people with consoles, that you could trade games with maybe? That’s usually a deciding factor, “what’s everyone else got?”
For Karaoke there’s SingStar, but do yourself a favour and get a later Rock Band / Guitar Hero title. Far more fun than it’s any right to be, and you can jam on a stupid plastic guitar whilst she warbles away, fun for all the family, less so for the neighbours.