Hi there Sasha! Thanks for bringing your question to us here at planDisney.
Since you mentioned you're unable to enter the park until 4:00 pm, I am guessing you must have tickets to
Mickey's Very Merry Christmas Party. Although the party doesn't officially begin until 7:00 pm, Guests may enter
Magic Kingdom Park as early as 4:00 pm with a party ticket and enjoy a few attractions or a meal before all the festivities start. If you are planning to visit Magic Kingdom Park earlier than 4:00 pm, you would need to purchase a day ticket in addition to your party ticket, and also make a
Disney Park Pass reservation for Magic Kingdom Park.
You might wish to keep checking to see if you can find a reservation for
Bibbidi Bobbidi Boutique for later in the day. Other Guests' plans change all the time, and it is possible someone else may cancel a reservation. I know that this experience has been incredibly popular since reopening, and reservations are hard to come by. On nights when Mickey's Very Merry Christmas Party is held, Bibbidi Bobbidi Boutique closes at 4:55 pm, so that would be a rather small window to try to find an available reservation in the 4:00 to 4:55 time frame.
There is so much holiday magic happening during Mickey's Very Merry Christmas Party! I attended the party a few days ago, and between the parade, fireworks, holiday photo opportunities, treat stops, and attractions with special holiday overlays, my group and I had a hard time squeezing it all in! I'm sure your niece's birthday will be magical even without a visit to Bibbidi Bobbidi Boutique. My recommendation would be to focus on the special party experience and perhaps save a visit to Bibbidi Bobbidi Boutique for another day.
If you think a royal transformation is crucial to her special day, you may want to check out the Ear for Each Other group on Facebook. There are several folks there who offer princess and other makeover services and will come to your hotel or home. I have read many glowing recommendations for their services, although I don't have any personal experience.
Sasha, I am wishing you a wonderful visit! Please let us know if we can answer any additional questions.
Anne Marie