Why is the actual issue sailing over so many people's heads.
OP is not looking for a way to go without DSC.
OP is quite rightly looking to prevent pay an arm and a leg for flights, park tickets, etc for a child who usually doesn't want to go or the mother cancels at the 11th hour.
No OP, you are not unreasonable for being wary that you are basing this on past experience, because this is a hell of a lot of money that seemingly as a step mum, you have no opinion on being wasted.
If at the 11th hour DSC was actually ill, then of course, it would be upsetting, but out of everyone's control. But this is not OPs concern.
OPs concern is that her past experience is that the mother cancels at the last moment, or DSC says she doesn't want to go. And no, OP shouldn't just blow thousands on an extra place for 3 weeks that she is pretty certain won't be used, because she's a villain if not.
I'm not sure how you do this OP, but essentially, you book her place if you can be sure (actual emergency aside) she will be coming.
I'd have zero problem paying for/taking DSC, which is what I can see you have no problem with either.
I'd have a massive problem if I spent thousands on DSC place, and then as we were on the way to pick her up, the mother called and cancelled.
And that's what has been your experience so far.
Again, I'm not sure how you do it, but somehow, you need to be able to recoup the cost, when the likely last minute "cancellation" happens.