Do you use insulin? If so, you can eat the desserts that you want and cover for it with insulin. It's the carbs that you're counting, not the sugar. Most sugar-free options have just as many carbs, if not more, than the regular options do. Also keep in mind that with all of the walking at WDW, your blood sugar will probably run lower than it normally does. Be sure to bring some candy with you in case of lows.