My son has a dairy and nut allergy and the cruise has always made excellent deserts for him. He particularly likes the flourless chocolate cake, which is dairy and gluten free. They also have a non-dairy whipped topping which he really enjoys. They also carry various flavors of soy and rice ice cream.
Don't forget to complete the medical form prior to sailing. Also, you can ask to meet with your head server prior to the first dinner to plan out her menu for that night. What we do then is after every dinner the head server would bring us the menu for the following day and we select a meal for him that is consistent with what everyone else is having, but specially made. We also special order breakfast and lunch for him at dinner the night before, which you can pretty much have sent anywhere you want to on the ship, including at Castaway Cay.
Once the dining room is aware of allergies and any special dietary needs, they bend over backwards to make the food enjoyable and safe. Moreover, everyone on the ship receives training on accommodating and addressing food allergies. What we love about it is that once we are on the ship we know the food is taken care of, so we can relax.
Ricky