My nephew has serious food allergies including dairy. My sister has it noted on every ADR when she makes them, then she calls X number of days in advance to confirm they have it noted. When she gets to the restaurant, a chef will come out and discuss the allergy with her and then he goes through the buffet with my sister and the allergic child to explain exactly what on the buffet is safe and not safe. I know that on occasion, when there is nothing safe for him to eat on the buffet (it is amazing how many things contain milk, butter, and cheese), they will make something special for him in the kitchen and bring it out. We have vacationed with my sister's family several times, and I am always amazed at how well Disney handles allergies in restaurants.
Now, my husband is allergic to peanuts, but he doesn't want a big deal made about his allergy, so he doesn't ant me mentioning it when I mke ADRs. He can usually tell what he needs to avoid.