Hi, Krista! My best advice for the theme parks on Christmas day is to pick one and arrive as early as you can, with plans to stay put. In years past, there have been instances on holidays where a theme park fills to capacity, and once it's full, you won't be able to enter, even if you've been there earlier in the day.
The Magic Kingdom is typically the busiest, but I also think it's the most festive. There's just something special about being in the Magic Kingdom on Christmas day! I'd recommend Liberty Tree Tavern if you're looking for something close to a holiday meal experience, but my favorite would be the Crystal Palace.
If you choose Disney's Hollywood Studios, you might enjoy a meal at the Hollywood Brown Derby. Thte food here is always wonderful, and it's almost an upscale feeling. If a buffet is more your type of meal, Hollywood and Vine has a nice variety of food that will suit just about any palate. The park will still be super busy, but I think you'll encounter a handful less people here than at the Magic Kingdom.
I want to stress that you make your reservations as soon as you can! Many restaurants book up for holiday meals, and I'd hate for you to be disappointed! You can book most dining experiences online by going to
Disney World Dining.
Have a great vacation!