If you need to do this because of cost difference I understand but if you have the means to get the dress then I suggest you get the dress! My DD (7 at the time) floated on air after her appointment and being in a dress. She smiled big the whole time and waived like a princess to everyone. The "real" Disney princesses ate her up at the meet and greets, she got more interaction than we normally do just because of her attire. I would suggest just bringing a change of clothes if she gets tired of it.