Hi MIchelle!
Your granddaughter is going to love the
Bibbidi Bobbidi Boutique (BBB)! She will be transformed into a true princess, with beautiful hair, painted nails and glittering makeup.
My daughter also loves to dress up in a different princess dress each day of our visit, so I can emphasize with wanting to maximize your value for the princess dresses you purchase for her. You can absolutely buy the princess dresses before-hand and bring them with you. I've found several princess costumes at local consignment sales, which save me a bundle! One idea that may work for you, is to purchase a few costumes ahead of time on consignment, online or at the Disney store, and then buy her one special costume during her BBB experience. The costumes at the WDW parks are different from those you find at local costume shops or even the Disney store at the mall, so you may want to spring for one special dress for dining the the princess and experiencing BBB, while having other, less expensive, costumes on hand for day-to-day park hopping.
Please let me know if I can help you with anything else!
Magically,
Stacy