Hi Karen,
Bibbidi Bobbidi Boutique is a fun experience, especially for little princesses. It's also extremely popular! While there is a possibility that you may still be able to get in due to last minute cancellations, don't base the success on your entire vacation around that one event.
I think you're doing the right thing. Keep calling. Also, be willing to take any time at either location. Your chances may increase if you are willing to get the girls in separately - possibly on different days. I know it will take more time out of your vacation, the chances of one person canceling are better than a family with two children cancelling.
Have you considered
Pirate's League? I know it's not a princess, but they do a great Mermaid makeover that could be equally as much fun. If that fails, the face painting stations throughout Walt Disney World do a tremendous job. I know my little girl looks forward to that on almost every trip.
Hope this helps, and my fingers are crossed. Have a magical vacation!