Hello Marilyn and welcome to the Disney Parks Moms Panel.
This is such a great question to ask Marilyn as quite often us ladies don't want to be encumbered by a large and somewhat heavy backpack or bag whilst enjoying the magic of Walt Disney World Resort.
With this in mind I'd like to recommend to you a
GymSack style bag as an alternative to a bulkier backpack. I find
GymSacks are not only smaller and lighter to carry, but also super easy to access with their draw-string type top opening. I'm sure that a quick
Google search for
waterproof GymSacks will provide a good choice of bags to purchase for your upcoming Disney trip.
I can totally understand your concerns for your dear grandson's health and wellbeing as one of my own children is an asthmatic and we always carry his inhaler with us wherever we go. I'm happy to share that of course
asthma inhalers and EpiPens are medical items which can be brought into the Disney Parks.
To try and keep the weight down in your bag be sure to pack a plastic, screw-top water bottle which can be refilled
for free at any water fountain. Plus you may wish to consider
Locker Rentals to store any non-emergency items; for Guest convenience lockers are available to rent for a small fee in each Disney Theme and Water Park.
Wishing you and your grandson an extra magical visit to
Walt Disney World Resort. Have a great Disney day.
~ Jane