I've seen some girls so uncomfortable wearing dresses at Disney. It's sad really; like the child thought it was a good idea, then the parents didn't bring comfy clothes to change.
Most girls do not dress up at Disney, though. It is a very small minority.
What about just doing a tee shirt?
Or bring a dress for an occasion, then change after to be comfy, if the child is insistent. Thing is, kids at a young age change their likes quite frequently!