My suggestion would be to check menus on the WDW website. There is a Special Diets team that you can contact as well.
For table-service restaurants it is important to note your dietary preferences on the reservation form. In this way you give the food service staff at the restaurant a heads-up regarding your needs and time to prepare something “off-menu” for you. As for the CS eateries, checking with a CM at the counter is your best bet.
I’m not totally sure about the Candy Cauldron, but I’d be surprised if no-sugar options aren’t available. I have seen no-sugar ice cream options at several of the ice cream spots.