If you want to make special requests regarding medical conditions or allergies for the dessert party, call WDW-DINE and make sure it's noted on your reservation. The desserts are not prepared in the same area where the party is held - they're brought in from somewhere else. If they know ahead of time that you need sugar-free desserts, they may be able to bring some in, but as noted it probably won't be a huge selection and you may not get all you can eat. If you don't mention it until you get there, I don't know what happens, but if they don't have the special items on hand they'd have to send for them and you might have to wait.