France has two sales per year and the dates are regulated by the government. I believe the dates are different in each city. In Paris, they are in January (right now) and July. I don't have the exact dates so I am not sure if you will be able to take advantage of the July sale before you depart. As for your daughter's dress, one time when I was in Paris, I observed an American grandmother purchasing a prom dress for her granddaughter. They were in Paris together and had so much fun finding this dress. They found it in the Marais area in a botique and they said they paid 200 euros.
Your daughter is going to enjoy Monoprix, Galeries Lafayette, Au Printemps, Bon Marche', BHV, H & M, and so many more!
If you girls have fun shopping, then just set out on foot and have a blast! There are so many places to shop for shoes, bags, and jewelry and you don't have to spend a fortune. Of course, there are the very expensive areas, but if you look at the department stores and boutiques you can find some great deals. I always find really fun things that we don't have in the U.S. and love wearing it all because it reminds me of my favorite city

I also love being able to have these things because they are unique here.
Do you have Suzy Gershman's book, "Born To Shop Paris?" If not, you might want to get it because it is helpful in planning your shopping excursions. It will also answer all of your questions about the VAT. My copy of the book has writing all over it from all of my notes from when I am in Paris. It give the hours of the stores, locatation, where you can find what, etc. Suzy is a friend of Bonjour Paris and mentions this site in her book.
Be sure your daughter has comfortable walking shoes! If you want her to keep up with you and have fun, that's a necessity. If you have any other questions, feel free to ask.