There's a wonderful fondue place just off International Drive called The Melting Pot (
http://meltingpot.com/). Lots of fun, and the kids would probably have a ball dunking and cooking their own food. (Of course, supervision is required, so may be more than you bargained for!).
You could also go to any of the restaurants at the various resorts; this doesn't require admission and they are all kid-friendly. If you're looking for something signature, try Jiko's at the Animal Kingdom Lodge or California Grill at the Contemporary (you can also watch Wishes from there).
There are also all of the restaurants along CityWalk near Universal; no theme park admission required.
http://www.universalorlando.com/citywalk/restaurants-club-comparison.html. And of course, there are the restaurants in Downtown Disney.
Happy birthday to your FIL, and have fun!