I am totally confused by the responses here. You live together, you are about to be married, which means that both of you are prepared to take the step to be not just emotionally committed but financially, yet when you tell your OH that you won't be able to join him and his kids on holiday because you can't afford it, he doesn't suggest to help?
I'm very independent and always to pay my way, hence always working FT, even when the kids were babies. OH and I have separate accounts and we like it like this BUT before we moved in together, we agreed that the fair system was that we each paid bills to the amount that meant we were left with a similar disposal income. That meant that he had to pay more and that his disposable income was less than it was before he met me, but that was part of the commitment. He doesn't have children, I have two, but holidays costs have always been split in two if we all got together.
I can't see a future wealthy marriage where he will continue to take his kids on holiday whilst you and your kids don't because you can't afford it. How can you avoid starting to feel resentful? Surely if that's how he conceives his life and relationship, you should have remained boyfriend/girlfriend?
The ex situation is irrelevant, it was agreed and that's how it is, but unless the reason why you don't have any money to go on holiday is because you don't budget your disposable income to save for it (after all, you and your kids could opt to spend money on designers clothes and accessories whilst he and his kids are happy with Primark), he should accept to contribute so that you and your kids can join the holiday (since clearly it's not a matter of him/them wanting to go alone).