I say if they really want to wear them in the park, let them. Where else can they dress like a Princess out in public. Just bring a change of clothes in case they change their mind. Also, draw the line at the shoes-- you will see many little Princesses in tennis shoes at WDW. Although I didn't plan on DD wearing her costumes in the park, on DD's 1st trip, she wore her Cinderella dress to the Princess meal the second day of our trip and got so much special attention that she refused to take it off the rest of the trip. That dress could stand by itself by the time we got home! (I did get her to wear Minnie one day instead). On DD's 2nd trip, she mostly wanted to wear dresses only (not pants or shorts), but not costumes (the exception was CRT where she wanted to wear her Jasmine costume and left the top and crown on with a skirt instead of the pants the rest of the day). I kind of missed the fun of her being dressed up. Enjoy it while you can!