Ideally, I do this in the am so they can wear thier do's all day. If you want to know, calling WDW would be best.
I'd also say, in our family, the $50 price tag is kind of steep. Call me cheap, but for less money, I can do the hair myself, get glitter hairspray (look in Halloween costume stores, or girlie stores or maybe CVS or Sephora) Sephora actually has all kinds of semi-subtle glitter make up. You can get her nails done before you go to WDW at a local salon for about $15, maybe less.
(We've done this as adults with our family....we call the glitter hairspray pixie dust..one can and everyone in the family can have glitter hair every day of the trip!

the guys don't tend to go for it too much though!

Bagh! )
Dresses can be more difficult but again, Halloween is around the corner, so Dinsey princess costumes aren't hard to find this time of year. Try
Disney Store, online DS, Target, Kmart,
Walmart.
Beyond that, you can find all the special hair clips and fake hair on sale in the WoD store in DTD (one of the two places they have BBB.) I don't know about the place in the castle, they might ahve stuff for sale there. Probably. Next door to where they do the updos is Tinkerbell's Treasures giftshop that has long been aplace to get princess stuff. They also sell this stuff elsewhere at WDW, but that's one place I KNOW has a great selection.
Incidentally DO look around Tinkerbell's Treasures...you'll eventually find Tinkerbell going around the shop. If you are the first guest of the am, they let you 'wake up Tinkerbell,' or at least they used to do that.
All in all, maybe not as magical as doing it at WDw, but it depnds on your child. Some salons won't do children manicures (insurance? state laws?) Some will do kids for free.
Oh, one more thing....at WDW individual resorts sell temporary tattoos...you can find these at your local party favor store for much less, sold as party favors. Once, in the WS of Epcot, in the Japan gift shop. I got this really cool temporary tattoo glitter paint. It came with a stencil, and I still have teh tube of paint like five years later. It only cost about $5 a tube, and each tube came with a stencil. I have used it a number of times. The tube is starting to dry up, but I REALLY got my money's worth out of it. The 'tattoos' came out REALLY pretty too. They last about four days. One stencil was a butterfly, the other a round design. I got pink and purple. I have looked for it since, and found it again, but I will look again next trip.