Hello Martha!
Thank you for visiting the Disney Parks Moms Panel. You intrigued me with this Sitpack, so I had to google the product to see what it was about.
It does not appear to be on the list of
restricted personal items, because it's not a folding chair, and it can fit in your backpack. The website states, "Please use your best judgment and do not bring anything that may be potentially harmful or disruptive."
A potential issue I could see is if it impedes the line flow. Many times while waiting in line for attractions at Walt Disney World, the line is consistently moving, so you don't have time to sit for too long. However, I can see the benefit for waiting in line for shows if you arrive early and are waiting for the doors to open. Use your best judgment and know that a Cast Member could ask you to put it away.
If I feel myself getting tired, I'll stop and take a break, grab a snack, and sit down and people watch. Or else I'll go on the
Tomorrowland Transit Authority PeopleMover which is one of my favorite relaxing rides.
If you have any other questions, please come back and see us real soon. Have a magical vacation!
Tania