My 6yo DS is milk, egg, and nut allergic so we probably see a lot of the same desserts. Ice cream, ice cream, ice cream....
Are you getting anything with the ice cream? My DS usually gets chocolate sauce (Hershey's) and sprinkle with his. We have a 4yo with no FAs so I was worried on our last trip about her getting fancier desserts than DS. Well, the regular kids menu doesn't have very many choices for the "regular" kids either. Standard is a make your own sundae that consists of a dish of vanilla ice cream and small cups of sauce, sprinkles, and M&Ms. The only thing lacking from DS's dessert is the M&Ms.
One thing I do as "insurance" is carry a small Betty Crocker sprinkle bottle in my bag filled with some kind of ice cream topping. Chocolate chips, sprinkles, safe candies of any sort. Just in case all they can offer is plain ice cream. I've also used a small snack bag of mini Oreos as a back up topping. I have rarely had to use them, but they take up little space and give me peace of mind.
As long as your expectations about what they have on hand are realistic, don't be afraid to make suggestions.
Another dessert we ask for to mix things up is a root beer float. Every restaurant has root beer on tap and they almost all have tofutti or rice dream ice cream available. Nobody has ever offered it up without us asking, but they've never turned me down when I've asked if it was possible. DS LOVES them.
We also eat at Chef Mickey's every trip and they stock top 8 free cupcakes so that's at least 1 non ice cream dessert in the mix.