We did a 12:50 seating. We stayed at GCH, so after our morning in the parks, we went back to the hotel and DD changed into her princess dress. Right after the lunch, we went back to the hotel to nap, so we had no issue with touring the park in her dress.
I don't know what time the parks will open during your trip. They opened at 10 for us, so we had about 2 hours in the parks before we headed back to the hotel to freshen up for lunch.