We're also going in December, DD will be 6 and DS will be 3! Maybe we'll see you there...
In Epcot my DD LOVES the Biergarten restaurant, mainly because of the oompah band and polka dancing. She talks about it all the time. Great for younger kids because it's a high-energy, noisy place - you don't have to worry about their behavior.
At Magic Kingdom, the Crystal Palace character meal was a big hit with our whole group. My kids also liked the Liberty Tree Tavern character dinner (during Halloween Party, not sure if there are characters at dinner during non-party times).
In MGM the Sci-Fi Drive-in was fun, they really liked eating in the cars.
At Animal Kingdom, some kids like Rainforest Cafe. I'm personally not a fan, but if you don't have one near you at home it may be worth looking into. I've only eaten at the one at Downtown Disney, but have heard that the one at Animal Kingdom is a little bit better.
My kids also love 'Ohana, especially all of the great activities they have for kids. We've only done the dinner there, but are looking forward to trying the Lilo & Stitch breakfast in December.
Have a good time!