Hello again, Kim!
Thanks for directing your follow-up question to me. It's great to hear back from you! I love helping Guests plan multi-generational family visits to Walt Disney World Resort!
To answer your question, technically no. Only Guests actually staying at Disney's Animal Kingdom Lodge or Villas are permitted to use the pools. But I do have a few alternative suggestions for you.
One of the nicest things about Animal Kingdom Lodge and Villas, in my opinion, is the amazing variety of cultural programs and recreational activities offered at these sister resorts. Your in-law's 9-year-old daughter can choose from many fun (and educational!) activities, such as Medallion Rubbing in the Jambo House Lobby, African Face Painting and "Inkoma" Animal Stamping at The Mara, Cookie Decorating at Boma, a Cultural Safari tour, Night Vision Animal Viewing (with special infrared binoculars!), Animal Tracking, and listening to African Folktales and learning to play African instruments. These activities are all complimentary!
If you would like to join your in-laws for a grown-up night out together, I highly recommend
Simba's Cubhouse, the supervised Children's Activity Center at Disney's Animal Kingdom Lodge. This is a super fun way for kids to spend an evening. Kids ages 3-12 can enjoy themed arts & crafts, games and movies as well as dinner and evening snack. Simba's Cubhouse is open daily from 4:30 PM to midnight.
My nearly 8-year-old daughter is a very BIG fan of the Children's Activity Centers. She would sleep there if she were allowed to!
I hope these suggestions are helpful to you, Kim. Please let me know if I can assist you further in any way with your planning.
Wishing you a magical vacation!