Hi Tonya!
I've taken my daughter to Bibbidi Bobbidi Boutique a few times and she was thrilled at each visit. I tell her since she's already a princess, it's OK to actually where her royal garments from time to time :-)
The dresses are the same as the one's you'll find in the Disney store or department store that sells Disney licensed apparel. December is normally the time we visit Disney, and they unfortunately don't change the dress style for that period (such as a special edition holiday dress). JC Penney sells these really elegant princess pajama dresses that I've seen some use as their Boutique dress.
You may want to purchase your dress from the
Disney Store or off-site anyway. You'll save a few dollars, plus are guaranteed to get the right dress in the right size (at the Boutique, they have a hangar of dresses but not always all the sizes for all the characters).
Hope this helps. May you and your princess have a magical time!