You can book BBB 90 days out like all other ressies these days.
I think your girls will love it. You are never to young or old to want to be a princess.
There are 2 locations. One in the castle at the MK and one in the world of
disney store in DTD.
There are three different packages that you can choose from. This is copied from WDW site:
Girls 3 years old and above can choose from 3 hair styles Fairytale Princess, Disney Diva and Pop Princess in 3 available packages:
Coach Package includes hairstyling and shimmering makeup starting at $49.95 plus tax.
Crown Package includes hairstyling, shimmering makeup and nails starting at $54.95 plus tax.
Castle Package includes the Crown Package plus the Imaging Package (one 6" x 8" and four 4" x 6" photos in a princess-themed photo holder) and complete costume of your choice with accessories starting at $189.95 plus tax.
The optional Imaging Package includes one 6" x 8" and four 4" x 6" photos in a princess-themed photo holder.
The World of Disney Store location offers a wide variety of beautiful accessories including ponytail hairpieces and tiaras.
We usually bring our own costume and DD gets her hair and nails done. I now bring a mega hold hairspray with me for her fine hair as the spray BBB carries is a very fine mist.
We like the location in DTD better as they offer way more in the way of choices for accessories than the location at the MK.