I wouldn't let my DD wear her dress all day in the parks. So we just put it on for the breakfast, over her clothes and then put it in the bottom of my carriage after. I suppose if you don't have a carriage then it would be a pain to carry around. Although the Disney dresses do squish up small.
One positive of having them in the dresses is that when the princesses come around they always compliment them on their pretty dresses. And my DD was dressed like Aurora and she made a big deal about them being dressed alike. It was really fun. And my DD loved it. She was only 3 last year when we went so you may need to ask your DD how she feels. I hope she's not too old!
Have fun!