It does kind of depend on how casually the comment of you maybe not going was. If it was said in a light and breezy, "Well, im pregnant so small chance i won't go, but not likely as i've always managed fine!" and all other communication throughout about planning it hasn't been brought up again, and spoken about like you're going, they probably felt secure that you would in fact be definitely going. Things have disclaimers all the time that almost never happen anyway, they've probably only really considered the scenario where you are there, or it was mentioned so casually they thought it might have been a joke.
They almost certainly didnt actually expect it to play out any other way than both you and your Dh going along and being in charge of caring for their daughter. Maybe it has nothing to do with DH being male, and its the sheer fact that there were going to be 2 guardians, and now there's only 1. Maybe their daughter has already started puberty early and they're not comfortable with her not having a female to help her with those needs? Do they know your DH well, or are you the main connection, the one they see at practices etc? It might of course be purely the fact he's male and will be the sole guardian that they have issue with, whatever anyone else thinks of that, its how they feel, and they have a right to feel however they like about who they are or arent comfortable with caring for their daughter.
Assuming the £1000ish they have paid out was done so on the reasonable belief you were going, i can see why they might feel you owe them something, because if it was made clear from the start and throughout it was a significant chance you might not go, rather than one casual throw away comment, they may have never agreed and wouldn't have paid out on flights, tickets, passes, entry to the event etc.
£1000 is a LOT to lose, maybe they already tried looking in to getting refunds and can't, neither of them can go in your place (plus surely it would cost even more in lost pay, and another flight etc) or they just cannot afford that option. They probably do hold you responsible as they made their decision to agree you to take her and paid out on all of this, believing you would almost certainly be there. Maybe they know asking you for the money (all, 50-50 or anything at all) is cheeky as all hell, but you have put them in a really awkward position of lose £1000 and have a very upset child, OR gamble their daughters safety for whatever reason it is they are uncomfortable with just your DH taking her.
Morally, only you know how clear you were or weren't about how likely it would actually be you could drop out. You don't want to pay anything towards what they're losing, but surely you must have some sympathy of how much THEY are losing because of your choice to not go along anymore. In your position, if you really weren't explicit that it was 50-50 if you went or not, i would feel guilty and like i had some moral obligation to reimburse at least something, but then i would never have offered to take anyone else child in the first place.