Michelle,
If I were in your shoes, here are the two that I would work on getting above all others;
Chef Mickey's - Breakfast or dinner, but I prefer breakfast, simply because I think that breakfast foods seem to work better at buffets. Either would be fine though. Go with whichever meal is the one where your boys rely on most for their energy.
Donald's Safari Breakfast - The buffet at Tusker House in the Animal Kingdom park may well be the best buffet food at Walt Disney World, and the theming is quite good. It is my daughter's favorite restaurant.
If they are into the Disney Junior characters, I would also recommend
Hollywood & Vine for breakfast or lunch. And if they like Winnie the Pooh then the
Crystal Palace is a good choice, though I find dinner is not as nice as breakfast or lunch.
If I can help with any follow-up questions, feel free to send them to me from my bio page!
Have a magical (and delicious) vacation!