Definitely just see what is available for your party, first! How many people? We did Thanksgiving with 15 people in 2004. We all ate at Cinderella's Castle, but we did it during the firework show. So it was a late meal, but we all ate together. And watching the Fireworks through the stained glass was quite beautiful.
Are you looking more for the best Thanksgiving food, a place best for family, or atmosphere? Top family restaurant for us, hands down, is O'hana! It's fantastic for the family and SO MUCH FOOD. This is another one we've done during the MK fireworks. You can see them, and they play the music in the restaurant that goes along with the show.