Hi Lucy, Your daughter is at the perfect age to enjoy Bibbidi Bobbidi Boutique...my little girls absolutely love going there. That being said, I wanted to share with you the "cheaper alternatives" that we have always done. We've gone several times (paying for 2 little princesses at once). What I do is I have them choose which Princess they want to be before we leave home and I bring the costumes with me. I either purchase them at the Disney Store online or at a local department store. Then, I pay for the "Crown Package" instead of the full blown "Castle Package". See:
HERE. They are still able to get hair, nails and make-up done and have always walked out of there feeling like royalty. Another idea that I did last time, was I brought new cotton sundresses instead. One daughter wore one from the Disney Store (with Sleeping Beauty on it), and the other chose a Minnie Mouse one. This was perfect because the girls got a lot more use out of the dresses (wearing them throughout the summer, instead of just for playing dress-up and Halloween). In addition, they were able to keep the dresses on all day comfortably as opposed to having itchy costumes that they wanted to change out of 1/2 way through the day.
Truly, Bibbidi Bobbidi Boutique is SO fun for little girls...I think your daughter will cherish the memories from the experience for a long time. With a little advance planning, you can pull it off without breaking the bank.
Hope this helps and that you have a wonderful vacation with your little girl!
Kaylene