Hello Kathleen! Welcome to the Disney Parks Mom´s Panel.
Bibbidi Bobbidi Boutique, either at Magic Kingdom or Downtown Disney, is a magical place for young princesses. As part of the experience guest can add a costume or other accessories to the selected package.
Availability of a specific costume cannot be guaranteed and though for different reasons, primarily because some may run out. Bibbidi Bobbidi Boutique is a really busy experience, especially at Magic Kingdom.
To make sure your little princess is not disappointed I second your idea of a plan b. My family always has a plan b, since my little Merida does not likes wearing costumes for more than 20 minutes or so.
Also consider that you will be visiting during a hot season, or may I say really hot and humid season.
It is awesome seeing our daughter immersed in a magical experience at the Bibbidi Bobbidi Boutique and showing off her new hair style for the rest of the day.
The Pirates League is another makeover experience that my family really enjoys while at Walt Disney World.
Glad to help you plan a magical Disney vacation.
Hope to hear more from you.
Miguel