For a fun atmosphere, I'd choose Liberty Tree Tavern or 1900 Park Fare. LTT's characters dress for the occasion, & the restaurant is homey to me. Unfortunately, it's in Magic Kingdom, which will most likely be a madhouse that day. 1900 PF is a character meal in Grand Floridian, which is beautiful at Christmas.
For better food, I'd choose Flying Fish at Boardwalk Inn, or Artist Point at Wilderness Lodge. If you eat at FF, make sure to tour all the Epcot resorts to enjoy the holiday decorations. WL is also beautiful for the holidays & AP is a great restaurant.
In other words, I'd try to choose a place or location that's Christmassy.