Hi
Ginger!
If you could make it fit your budget, I would highly recommend staying at a
Disney Deluxe Resort Hotel in the Magic Kingdom Area, preferably one that is serviced by monorail transportation. If she gets overstimulated at the theme parks, a "monorail-resort" would be ideal. This would facilitate commuting back and forth between Magic Kingdom, or Epcot, and your resort hotel for mid-day breaks (naps, relaxation, swim, etc). However, if a Deluxe Resort is not an option, then I would suggest staying at Disney's Port Orleans Resort - French Quarter. Read about this resort
HERE. Another fabulous option to consider in the Moderate Resort category is The Cabins at Disney's Fort Wilderness Resort. This resort is also conveniently located in the Magic Kingdom Area. For other ways to make her experience at the Walt Disney World Resort even more enjoyable, please read
HERE.
I can completely understand your concern regarding her limited diet due to aversion to food textures. I would recommend contacting Disney Dining at (407) WDW-DINE when making dining reservations at table service restaurants. Please have the Cast Member note the intolerance to the offending foods. In addition, I recommend alerting the wait-staff at the restaurant about your niece's specific diet. You may ask to speak with the chef. You will find most are very accommodating and will often prepare off-menu meals specifically tailored to your niece's needs. Please let me know if I can answer any other questions for you regarding this matter. It would be my pleasure to help you plan a magical Disney vacation for you both!