At Flat Bread Grill in MGM, it was automatically jello. We didn't ask about a substitution.
At Tusker House, I asked if they could get the chocolate cake and was told no, kids meals come with jello only.
We all split up in Epcot. My daughter's kids meal from Lotus Blossom came with choice of an almond cookie or ginger ice cream. The German CS didn't have a kid's menu, so my son automatically got a "grown-up" dessert. He picked the Black Forest cake.
They had their choice of dessert at our resort food court.
The kid's dessert at Wolfgang Puck Express was a big cookie. In fact, the adults also had to get the big cookie because they were out of creme brulee. I was really disappointed with that. I didn't like my entree either (Thai noodle salad), and they said I didn't get a side or appetizer with the plan, so that whole visit was a disappointment.