Hi Sara,
I am so glad you came to us here, at the Disney Parks Moms Panel, with your dining question. We always hope to assist other guests with our personal experiences so that their vacations are a little less stressful.
Dining at Walt Disney World happens to be one of my favorite things to do. Don't get me wrong, I love the attractions, but trying new places and menu items is just as fun, in my opinion!
As the mom of three picky-eaters, I can tell you that the cast members we have encountered in the past have always been receptive to this and helped any way that they could. The chefs are also pretty flexible if there are dietary needs or concerns. I would say that if potatoes are available, they will most likely allow the substitution. However, I cannot guarantee this. Be sure to ask your server when placing the order. If they can allow a substitution and potatoes are not available, they will be able to tell you what other alternatives are possible.
Have a magical visit with your family. Bon Appetit!
~Allison L