Hi Kristina, what a wonderful birthday you have planned for your little Princess!
Bibbidi Bobbidi Boutique is so much fun, and is an experience that my littlest girls look forward to on our visits. The Magic Kingdom location of BBB opens at 8:00am. The appointments last between 30-60 minutes depending on the hair style chosen. Also, you may have to wait a little while for your chair if the make-over before you is running a bit longer than expected. If you are choosing, "The Castle" plan (which includes a complete costume and imaging package), you should definitely plan on at least an hour.
Based on our visits to the Boutique, give yourself an hour and a quarter or so between when you make your appointment and when you want to eat at
Cinderella's Royal Table. You need to check in for your dining reservations at least 15 minutes early. It's not everyday that you get Pixie Dust in your hair that sparkles like your brand new tiara! You don't want your little Princess to feel rushed and if she wants a little extra time to twirl around and gaze into the mirror...then she'll have that. If you still end up with a few spare moments, then you can take her on Cinderella's Golden Carrousel right behind the Castle.
Maybe make an 8:00 reservation at the Boutique and a 9:15 or 9:30 breakfast? I'm including a link to a previous answer in which I gave some tips to a guest for booking CRT. See:
HERE (in fact, since writing that, I was recently able to book it again for our upcoming October trip by doing the same thing mentioned in that link).
Wishing you a magical vacation!
Happy Birthday to your little girl,
Kaylene