Welcome to the Moms Panel Bethany!
Sounds like you are planning an exciting day for your little princesses. My kids are close in age to yours and I have found that my kids enjoy special experiences more when they are fresh and full of energy.
I would make a reservation as early as I could at the
Bibbidi Bobbidi Boutique (BBB) location at the Magic Kingdom and then a late breakfast or an early lunch reservation at Cinderella's Royal Table (CRT). That way they'll have plenty of time to enjoy BBB without being worried that you'll miss your CRT reservation.
Once you've finished at BBB, you may have some time to take pictures of your princesses around the Castle or in Fantasyland without too many people around so that you can get some good shots.
Visiting BBB first thing in the morning will give your princesses more time to enjoy their transformations all day. If you wait until later in the day, they won't get to enjoy it as long. If they are planning to wear their princess dresses, you may want to take a change of clothes for later in the day in case they get hot. Sometimes those dresses can get a little "scratchy" to a little princess' skin.
Please feel free to visit us as often as you'd like during your planning. It is fun sprinkling pixie dust on your trip!
Wishing you minnie magical memories :-) Allison