Hi Shawnda!
Glad to hear you visit and read the site often. I liken us to Consumer Reports, giving helpful Disney information to terrific guests such as yourself!
As a dad who has bought every different style Disney costume, nightgown, etc. for my now 6-year old daughter, here are my suggestions:
If you're referring to the Disney princess nightgown/pajamas, they are beautiful, but I would probably stick with the princess costume dresses. The nightgowns are flimsier, and more prone to getting torn. Most of the one's I've gotten for my daughter also drape to the floor to where she's dragging it when she walks (wouldn't want it snagging on a ride somewhere).
If you're referring to just the regular costumes they should be fine. But an evening in Florida in June is still pretty hot, which may bother your daughters wearing it, so keep that in mind. If you do this, you may want to bring a change of clothes in case they want to change out of it.
Hope this helps! May you all have a Bibbidi Bobbidi Beautiful time!