Ah I hope you can figure out what the issue is, I'm sure the understanding would help her feel like she can go.
This is a long shot, but is it possible that the date you have booked falls into a themed experience she is not as keen on? At HP Studios, there is usually a theme going on, especially in the autumn, for example right now is the Dark Arts event and it says this on the website:
"Please note during our Dark Arts feature running until 10th November, there will be two processions of Death Eaters throughout the Studio Tour. The processions will use lighting and sound effects at the Leaky Cauldron, Dumbledore’s office, the Potion’s classroom, Hagrid’s hut and bike as well as the Hogwarts Express train and train tracks."
Is it possible that now the reality of being able to go is setting in, she is concerned or disappointed about some of the activities not being wheelchair accessible?
Does she access online communities or have online friends? Is it possible that while she wasn't bothered herself, someone has given her a hard time about going to HP Studios because of the JKR controversy and now she's upset and conflicted?
I do get what a conundrum it is; you want to figure it out so you can offer reassurance, but maybe listing out every possible reason for her not wanting to go, might just make her worried about things she wasn't even thinking about in the first place!
Do tell her, and if she is big into HP she may already be aware, that a huuuge proportion of the fandom are adults. I went when I was 21 and adored it, I'm 30 now and planning a repeat trip.
I guess if she needs care, the contingency will have to be that you all travel down, and if DD still really doesn't want to go, one of you sits with her and does something else while one of you and DS goes in.
Best of luck, hope it all works out