I think your emotions are confusing you. You started this thread by saying that you were upset that your OH wasn't prepared to go on holiday with you and your children without his children, but now it's about him not being prepared to take everybody because although he could afford to, he doesn't want to.
There are a lot going on in the dynamics of your family. Your ex who can't take your kids on holiday -which is not the fault of your dsc-, you who can't afford to take them - because you changed jobs, which is not their fault -, and the issue with the timing.
Your DH could resent you for not being able to go on holiday with his kids because you've changed jobs, and so could your sc for missing out on a holiday with their dad.
I think the real issue is that everyone has to accept that this year will mean compromises and they will all accept it, including your kids, because they love you and want the best for you, which is to find happiness in your new enterprise. Your kids are still young enough to go on a holiday abroad as a family, it's not too late and they already enjoyed a holiday nice holiday 3 years ago. How about starting to plan next year instead and start to put money aside?
The timing of your dcs going today and the envy that came with it is probably what triggered it all. Don't be jealous for your children. My kids step-siblings had to deal with the same, hearing about them going on very fancy holidays with me, but they have never resented them for it at all (and my kids would never brag about it). Instead, the eldest has started to work and saved her money so she could afford to go away with her friends. My kids might have had nicer holidays, but they don't have ipads or the latest mobiles (unlike their SS). Some of their friends do but they are not envious. My DD has also started working week-ends and is saving every penny for her holiday with her friends next summer. She's very excited at the prospect.
Very different upbringing, but they all get along great and there is no jealousy between them.