Hi Jaime,
You must be so excited for your upcoming vacation! A massage or spa treatment is a great way to break up some of your time spent in the parks. I'm assuming that you're considering letting your daughter go to one of the
Activity Centers while you get a spa treatment.
There are Activity Centers at 5 Walt Disney World Resorts: Animal Kingdom Lodge, Polynesian Resort, Wilderness Lodge, Yacht/Beach Club, and Dolphin Hotel. They generally open between 4:00 and 5:00, so you would need to plan for an evening treatment. Mandara Spa at WDW Swan & Dolphin Hotel is open from 8am-9pm. This isn't a particularly busy time of year, so you may be able to get in. Call (407)934-4448 for more information. If you do a search for "Mandara Spa WDW", you'll be able to access the spa menu.
If you're staying at Animal Kingdom Lodge, Boardwalk, Contemporary Resort, Coronado Springs, Wilderness Lodge or Yacht Club, you can get get massage treatments at their health clubs.
Now, another option would be for you and your daughter to go to the spa together. The
Spa at Disney's Saratoga Springs has a special treatment for children ages 4-12. To make reservations, call (407)827-4455.
There are a lot of options out there. Once you decide which option is best for you, if you need more help, feel free to submit another question. All of the "moms" on the panel want you to have an enjoyable and relaxing experience.
Hope this helps. Have a magical vacation!