Did you not see my two posts where I explain why I will be carrying an outfit for her?
It is very frustrating that people on this thread have questioned me on that. Trust me, if I didn't think I may need it, I wouldn't carry it. My daughter has special needs. This is something we have dealt with before.
I actually do have a book bag that I use daily. It will be the one I use. It's not overly big and its comfortable as long as it's not overly full. As I get closer to my trip , j will begin planning specific things I will carry to make sure I am not over doing it.
Thank you to those who have answered my questions.
Your first post said, "Any help will be helpful." I suspect that is what most posters read, and why they felt you'd appreciate suggestions on bag styles and what they prefer to carry.
Having read your further comments, it still may be wise to choose the extra outfits based on compactness:nylon shorts/undies/tank top. One summer trip, I saw a mom at a rest stop with a daughter prone to car sickness. She had the daughter prepped in a tank top, which was brilliant. Changing her top shirt was super easy.
[In your case, this may sound crazy, but something like an ultra thin Lightdays might be a great help. I know multiple young ladies who successfully used this method, and it works brilliantly. This challenge is rather common among all girls. Many face this challenge, even into middle school. ]
To go back to your original question, It is a matter of personal preference (where/how to stow). We've tried almost every type of bag available at WDW. As has been said, on most rides you keep your bag, but maybe are asked to stow it in a pouch (Star Tours). I dislike having straps wrapped around my legs (BTMRR).
Also, long delays at the bag check. Bag check guards make you unzip every pouch. I think that is also why someone suggested a see through mesh bag.
Last, as others have said, one helpful idea is simply to review your bag contents periodically. Remove items you don't need or aren't using. Add ones you do, or items that need to be replenished. Creativity also pays. Bonus points for items that do double duty, like Ziploc bags.