Hi Sunny!
A vacation with a whole family of princesses, love that! Going with a group with children of various age groups can seem daunting, but it doesn't have to be! My first suggestion is to let your older girls sit down on the Walt Disney World website or with a guide book and have them mark which attractions or extras interest them the most. You can create a family must-do list and ensure everyone is getting their say in the day. This way your teens can have time on Space Mountain while the little ones get to enjoy Dumbo the Flying Elephant and no one feels left out.
For your oldest daughter, depending on her interests, consider letting her have some "teen time" at DisneyQuest or perhaps booking her a
surfing lesson at Disney's Typhoon Lagoon. She may enjoy some time to herself.
As for dresses, your daughters can most certainly wear them whenever their hearts desire! My daughter was almost four on our last vacation and I couldn't get her out of her Princess Belle gown! She wore it over her shorts and t-shirt every day (I highly suggest wearing the gowns over light clothing or packing a change of clothes as the dresses can get hot or itchy after extended periods of time).
I hope this helps you plan your magical vacation, Sunny. Please come back and see me if you have any other questions.
Pixie Dust,
Ashley