Thanks for all the replies, I'll try and address some of the points brought up:
We really do not favour DS. I'm really careful to give them both time and I actually spend a lot of time making sure family life is arranged around DD as her needs are more complex and she has a lot of rituals and sensory issues that we need to accommodate. We made him go to something he wasn't keen on in the summer because DD wanted to go, and he spends a lot of time at home having to accommodate her rituals which do impinge on his life a fair bit.
The reasons we booked it now rather than years ago is that they are BOTH now into it. Previously it was only DD plus I thought DS wouldn't enjoy it and would be disruptive if we went, plus we generally don't go as a family to something only one person is into. So when I saw they were both into it and enjoying it, I thought it would be great to take them (even though I think DH and I will be a bit bored, potentially, but that's fine, we are going for the kids).
I'm very aware of all our additional needs and requirements and good at juggling and accommodating them all. DD won't be thinking about crowds etc as she knows I'll ensure she has what she needs. She knows I'm her advocate and protector, I've had to do a lot of fighting for her with schools and doctors.
People accusing me of being a bad parent for not watching HP are very far off the mark. I've spent hours listening to both my children talk about their special interests, watching TV I'm not interested in, etc. HP was the one I didn't get involved in much and DD read the books at school, and talked with her friends about it. But I know all about Pokémon and Hatsune Miku!
We didn't "do" HP pre age 10 for her at all. She wasn't allowed before then to read the books because there are themes in there that we didn't feel were appropriate and that aren't compatible with our family faith, and we didn't want the kids reading the books and seeing the films until they were old enough for us to have discussions around that. We also don't do Halloween for this reason.
So now DS is 10 they are both old enough for the discussions, and DD knows this. We've had very open conversations about all this.
I don't know if it's the trans thing. There was a conversation over a year ago when she said she thought JK Rowling was transphobic, but we looked at what JK actually wrote and said and she agreed it wasn't. And when she was all quiet the other day and I didn't know why she was quiet, I asked her if it was to do with that (running through a list of all possibles however tenuous) and she said no. I eventually got it out of her it was to do with the trip. So I don't know as she seems to have said it isn't that.
I actually said to her when she was unable to articulate what she was thinking, "does it help to know that it's booked so we are going?" as often it does help her to know there's no options, she finds making decisions really hard.
I am going to try talking to her again obviously. But this thread was about am I unreasonable to approach those chats with the stance that we are all going unless a very good reason is given (and I honestly can't think of what one would be).
The bottom line is that she was into HP until very very recently, I thought she still was until after the tickets were bought, we thought they would both love this trip, and it is booked and paid for.