Hi Wendy,
You are smart to be thinking about a stroller at the Walt Disney World Resorts with a five year old in the summer. We originally only rented a stroller for my 10 month old nephew at Easter last year and upgraded to a double for my five year old niece because of all the walking.
Stroller prices currently start at $15 but there are ways to save on that price. The strollers do not look like the ones in the picture and are hard plastic with no recline.
When we went in April, we reserved a stroller from Magic Strollers. While the first day was $15, we received a discount on multiple days. Plus the strollers reclined which was great to let my nephew nap. They will deliver and pick up from any Walt Disney World Resort.
As for the weather, it will be hot and humid with the afternoon thunderstorm so pack a poncho or umbrella. As for crowds, it will be busy as most schools are on vacation.
Wishing you a magical day!
Beth