I do believe OP doesn't want her SD to come. If she did, and the only issue was a very strong belief that the ex would cancel at the last minute, she wouldn't have brought up that she was already going, that she wouldn't enjoy the rides, that she would be sulking and ruining the holiday for them. She would also already have spoken to DSD and asked her how she felt and whether she thought that her mum might have an issue for it.
Every single one of those points is relevant as they increase the likelihood of a last minute pull out.
Dm wants her holiday to be the special one - she's more likely to pull her out.
Not wanting to go on the rides - Dm fills her head with the idea she'll have a terrible time because she hates rides and she doesn't have to go if she doesn't want to/doesn't support Ops DH in saying cold feet isn't an excuse not to go when tickets are paid for and it won't be as bad as she thinks. She gets pulled out.
She's miserable the whole time - winner for mum who can use it against DH. He took her on a holiday she didn't even want to go one, guilted her by saying they'd lose money and clearly doesn't care about her feelings. Should have let her stay and that's the last time she's backing him up.
None of those are beyond the possibility when dealing with someone that has been problematic. It really doesn't take much imagination to see why Op considers those details relevant, and that's without all of the backstory.
It's also not unreasonable to want to speak to DSCs Dm first rather than getting DSDs hopes up, watch them get dashed by her mum, and then be accused of deliberately alienating her. Not to mention the impact that will also have on DSDs relationship with her Dm. Adults discussing finances and making the decision is better than involving a young teen before it's even certain that there's an option that won't leave op burning £2k on an empty plane seat and unused park tickets.
Her dad, not the OP, should pay for her ticket, bookings etc. Explain to his daughter about the cost involved.
So he foots the entire bill for one of his 3 DCs but mum pays for the other two? Thats not treating his children equally either.
The issue isn't Op paying if she chooses to do so. The issue is OP's money being wasted by things which are completely out of her control.