I think you’re being a bit ridiculous to be honest, you didn’t want to go to visit your parents in the first place, but have accepted going if they and in laws split the cost of the flights to pay for you all to go.
the only thing your parents have really done wrong here is delay actually booking the flights, and not explaining why, they’ve said all along they are going to book them. Maybe they didn’t have the funds when you wanted to book them? But do now? Or maybe they were actually busy?
it all seems a load of dramatic nonsense over nothing. Why would your in laws refuse to pay half of the original agreed cost? Literally nothing has changed as far as they are concerned, to refuse to pay their original contribution, even though it was agreed, seems extremely petty and tight.
you’re behaving ridiculously IMO, if you didn’t want to go you should have just said no and meant it, if you wanted to book when YOU wanted to book you should have asked both sets of parents for half the money each upfront and booked it yourselves.. I’m actually amazed you didn’t do this but you’ve ignored every single suggestion of that on here.
and now you’re refusing to go, on what?, principle? Because they didn’t cough up and book it all for you in a timely enough manner, after all your pestering and deadlines. You just didn’t want to go to visit your parents at all, neither did DH and now you’re using this as an excuse to cancel.. so now in laws and kids miss out as well.. absolutely unbelievable.