If you've never done Chef Mickey's, you should go there first. Driving up to the Contemporary and walking around inside, well its as if you're participating in a cherished piece of Disneyana. Nothing beats the party feel of the place and the monorail traveling overhead for sheer enviroment.
That being said, Tusker House has a larger feel about it. More of a themed setting and a much better and larger selection of food. The characters are the same and they come around to party it up just as often. Not quite as loud either and your less likely to have other kids show up in your pics.
Kids are a ton of fun, they'll be happy either way!