Hi, Sharon! Walt Disney World is a great place to go if you have food allergies. I don't think I've seen anywhere so good about accommodating food allergies, and I've traveled with people that have wheat, dairy, and peanut allergies!
My biggest recommendation is that you note on your dining reservation that you have an allergy concern. This puts the restaurant on notice, and many times, they will address you as soon as you come in with regard to the issues. What I have seen done before at the Crystal Palace is that a chef or the restaurant manager will come and help you go through the buffet line to tell you what food items may be safe for your granddaughter.
You can also visit this website
Special Dietary Needs at Walt Disney World. They will give you some good advice on how to address the issue when you are at Walt Disney World - not just at the Crystal Palace, but to other restaurants as well.
I hope the restaurant staff ensures that you and your granddaughter have an absolutely magical, stress free visit to Walt Disney World!