What a nice idea!
A great way to make a birthday at WDW special is to send a wonderful gift basket to the room of the birthday girl. You can go to the Disney Florist and Gift Basket website at
disneyflorist.com and see dozens of festive choices. But don't worry if you don't see exactly what you want. The wonderful cast members at the floral department can also create a custom basket for you to almost any specifications.
You could also call Disney Dining at 407-WDW-DINE and make the young ladies a reservation at a particularly special and grown-up restaurant -- maybe the California Grill or Artist Point. You could then call the restaurant and leave your credit card number so that the dinner is on you.
Another fun gift would be a gift certificate to the spa at Saratoga Springs. The ladies could get mani-pedis, or the full massage and facial treatment depending on the size of the certificate.
You could also get them tickets to see Cirque du Soleil La Nouba. This a spectacular show that plays at Downtown Disney. Very special.
These are just a few ideas. I hope they spark your imagination on how to help your niece celebrate.