Hi Nicole!
Happy 3rd birthday to your little princess! Celebrating at the
Bibbidi Bobbidi Boutique sounds like a perfect way to start the special day! I would recommend trying for an 8:00 a.m. appointment. This way you have time before the Magic Kingdom opens (if it's not an Extra Magic Hour morning) and get the pleasure of walking down an empty Main Street, USA. There is something extra magical about being able to soak in the beauty of Main Street without the crowds. This a highly-coveted early-morning reservation, so I recommend booking it 180 days prior to your daughter's birthday.
Cinderella's Royal Table is also a popular character dining restaurant and experience, so I recommend booking that early as well. I would suggest booking a dinner time that is as close to the time that you would normally eat dinner. With small children, we tend to eat early at home and at Walt Disney World to attempt normalcy in schedules. Though when we took a couple of our daughters to eat in Cinderella's Castle, there wasn't a lot of eating going on. It was more ooh-ing and aah-ing as they were enthralled with the Disney Princesses and the majestic surroundings.
Another tip I would recommend is bringing a change of clothes in case your princess gets tired of wearing a princess dress all day. Depending on the weather, some of my girls love staying in their dresses from morning to night. Others do not and are ready for some shorts after a few hours. I just like to be prepared for either situation.
I know your princess will love her enchanted makeover! I wish your family a most magical vacation!
Tania