Aloha Sharon!
I just love multi-generational trips to Walt Disney World Resort. I still remember a trip I took with my grandmother forty years ago. I cherish my memories of that trip. I hope your family's trip is every bit as magical as mine was in 1974.
The showers located under the waterfall of the Lava Pool at
Disney's Polynesian Village Resort are only accessible to Guests during pool hours. For example, next month the Lava Pool will be open from 9 a.m. to 11 p.m. and thus, the showers there will be closed outside those hours. Please note that pool hours change from time to time so it would be good to ask what prevailing pool hours are for the Lava Pool when you check in.
Guests at Disney's Polynesian Village Resort do have access to the Fitness Center at
Disney's Grand Floridian Resort & Spa. (Your resort and Disney's Grand Floridian Resort & Spa are connected by a pleasant paved walkway.) That facility, and the showers therein, are open 24 hours a day.
Enjoy an amazing trip with your family, Sharon, and please have a Dole Whip at the
Pineapple Lanai for me while you're there.
Sending best wishes and pixie dust,
Jill