Hi fraggle, as you have gathered from all these posts, what you and your DCs are experiencing is quite common in step families, and also there is no easy answer to it.
Sadly, breakups always have the biggest emotional impact on the children. There will often be divided loyalties where an Ex takes on a new family with DSCs plus additional offspring from a new DP. your DCs are no longer his priority, cruelly he is making that message clear to you and delivering it a heartless way. I admire you for doing as best you can to mitigate the hurt and disillusionment your DCs must feel at their father's insensitive behaviour.
It feels to me like he is trying to "get back" at you via the children, because he is rubbing your nose in the fact he now has a "wonderful" life full of great activities etc blah blah.
They will indeed form their own opinion of their father as they mature into adulthood. It will likely make them better parents because they do have a wonderful role model of good parenting (in you). I believe I have become a better Step mum because I went through what your DCs are going through and it did have a lasting effect, but I got over that and have gone on to be determined to make life good for my DSS.
I guess the only thing you try to do is reason with you Ex by asking him to not tell you or the DCs when they go away. He needs to man up, stop acting like a childish brat, and face HIS responsibility for HIS children and try to see it from their point of view. Apart from that, being there for them (and resisting the temptation, as you have done, to criticise him to them) is all you can do, no magic wand Im afraid!