Hi Jennifer,
first up; are you aware of the fact that the scoliosis will be caused by the EDS? Just kaing this quick sidestep because -unbelievably imho- too many folks get misinformed on this subject and start to think 501 things are wrong, when in reality it's one diagnosis causing 501 problems.
There are many types and brands of wheelchairs out there that have height adjustable pushbars. Don't know how your DD is affected in daily life, but depending on that answer you might want to consider finding out if you can get a (partial) coverage for her own wheelchair. With EDS it is so much more important to get her a chair that FITS her. Any non fitting can result into problems. Too little support; chance of (sub)luxation because of the lack of support or her unconciously wanting to overstretch the joint -which at the moment of doing it can feel great, thus the unconcious wanting to do it-. Too much support can cause the chair to (sub)luxate joints. Armrests too high; can cause shoulder, back and elbow dislocations. Too low; same risk. Armrests out too far from her upperbody? She'll reach for them; again not the right way to move about. And the list goes on and on and on. If going down that path; help from an occupational therapist can be a great

, provided they either are in tuned with EDS or willing to learn.
There are some great chairs out there that are for one able to provide these options and not the least bit important for a child; can grow along with her. Esp. if you find you'll have to invest in such a (much) more expensive chair, you don't want to have to buy a new one ever year or two just because she grows. I know one of those is the childs chair from Colours (use their chairs myself, so that's why it pops up in my mind) but there are many more out there.
Again, I'ld try to find out what she might or might not be entitled to through your insurance. I'm of NO help in that department, am not from the US. Based on that, her needs and what you can afford, I'ld personally advice to look into the more expensive but better suiting chairs. With EDS you don't want to follow a 'oh she'll grow into it eventually in a couple of years' approach as that can cause problems on the longer run. For a one time use it's not
that bad, but a couple of days in a row at WDW could be a pain for her, depending on how precise her body reacts. That's something you might find will chance along the way, so also something to be on the look out for.
Cushions can also make a world of difference for her. Try out different types with her, it can be the difference between pain or not. And relatively easy and affordable to invest in. How did she do with bumps etc. while still using the stroller? A lot of bumping can cause pain and muscle fatigue for those of us with EDS. Simply put that shaking from going over bumps makes the body shake. Because of the EDS that body handle take shaking less than you'ld normally expect. Keeping the joints together at such moments can result in increased usage of the muscles, which in it's turn can cause fatigue, pain etc. In a lesser scenario bumps can even result in dislocations.
In the adult "chairworld" there are some amazing chairs to deal with the latter. There are a couple of brands that offer 1 or 2 point suspention. For me, those felt no different than a chair without. A fair amount of others with EDS do find it a positive difference. There is one brand (colours, US based brand) that offers 4 point individual suspention. World of difference! Amazing, basically. Unfortunately that's not available in the ped. chair world yet as far as I know.
How is her skin? If she's got any of the skin related problems that EDS can come with, the choice of fabric can also be of influence. Skai for instance can stick to the exposed skin in 'warmer' weather. Not what you'ld want if she's got a skin that easily tears. Cottons can be a lot friendlier on the skin, but downside is the time they take to dry after rain hit you. A second cushion cover and back cover can safe her comfort in those situations.
When testing out those height adjustable pushbars; also have her try them! There are some chairs that have them come so low, she could use them herself also if she can walk but needs something to hold onto to make it a bit easier. Sometimes can be a great way of still keeping moving but not overdoing it. If you do want to look into that option; also have her actually walk with the chair, preferably also outside. This way she can test it IRL so see if the chair operates in such a way she can also use it as a fancy walker.
One thing she'll find very important (and she's right

); the colour, fabric, basic look of the chair. Most ped chairs will come with the option of different colours. Depending on brand, type and manufacturer she could have 2 choices right upto "whatever you want is possible". Some options are more expensive then others, so keep that in mind before seeing something online, making a promiss and then finding out it might be too expensive.
Either way, I'ld rather look into doing some "IRL" buy, not an online one. IRL you can have her use it, see how it fits, if there are any things that don't suit her needs etc. Online it's always a gamble. You can take meassurements, but with a first time chair it is way too easy to slip up and end up with a chair that's not perfect for her. Whether you end up going for new or used, I'ld follow the "try before you buy" tactic to death. If you do want to look in new; know that some manufacturers will allow you to try out a chair at home for a couple of days upto weeks before making the purchase (or not). Mostly -unfortunately imho- the more expensive brands seem to do that. It wouldn't allow you to try out a custom built something for her if you'ld find she needs that, but it will give you a good idea whether or not the base concept could work for her.
Hopefully I didn't just make you go "help, what have I gotten myself into!?"
