Hi, Kevin!
It's wonderful that you've planned some special princess-themed experiences for your upcoming vacation. I have to tell you, though, that your two reservation times are just too tight, even with using your own car for transportation. The magical transformation at Bibbidi Bobbidi Boutique at the World of Disney store in Dowtown Disney will take
at minimum one hour. Then you'll need to factor in travel time to the Magic Kingdom Park. If you are driving your own vehicle, parking will be in the Transportation and Ticket Center lot. From there, your family can hop on the monorail or take the ferry over to Magic Kingdom. Unfortunately, it's just not possible to have a royal makeover in Downtown Disney and travel to the Magic Kingdom Park in a 60-minute time frame.
I suggest you think about which experience - a princess makeover at Bibbidi Bobbidi Boutique or Fairytale Dining at Cinderella's Royal Table - is more important for your family. Once you've made your decision, keep the priority reservation and cancel the other one. Personally, I think dining in Cinderella Castle is a really magical experience, one your princess will remember forever. The dining room with soaring vaulted ceiling, royal banners, Gothic decor and themed stained glass windows is a wonder to behold. And it's quite a treat to be able to meet several princesses during your meal. Even without a makeover at Bibbidi Bobbidi Boutique, your princess will still feel like royalty in her favorite Disney princess dress (and tiara, shoes, wand, etc.).
Whichever princess experience you choose, I hope your family has a magical time at Walt Disney World Resort!