Hello Scott,
Thanks for stopping by planDisney with your question. Before you know it, you'll be joining
The World's Most Magical Celebration!
A visit to any Disney theme park includes a lot of walking and a lot of standing around (in queues, while waiting for parades, and while viewing nighttime shows). I can see why a collapsible stool might come in handy; however, even though it is small enough to fit into a backpack, I think it probably falls under the category of folding chairs. Those are on the
list of prohibited items in any theme park, water park, Disney Springs, or Star Wars: Galactic Starcruiser.
There are plenty of benches and seating areas located throughout the theme parks for guests who just need a rest. If you have
mobility issues, you may consider
renting a wheelchair. Rental locations can be found near the entrance of each park and in Disney Springs where you can get either the traditional push type wheelchair or an
electric scooter. You can also bring your own walker with a seat attached into the parks, but with all of the walking that you'll be doing the wheelchair or scooter might be more relaxing.
Hope this helps. Have a magical vacation!
Diane