Actually NAR4 - I think you did exactly the right thing.
You have a relationship with these parents, so they invited you first. Personally I think passing them on to your Ex is bad form - putting your friends in a bit of an awkward position. It's for you and your ex to decide what you do and manage it.
You SPOKE TO YOUR EX FIRST (I think some posters are missing this bit) and then agreed attendence after you had checked with him. This is the right thing to do, working together to agree changes. Sounds like he's being an arse.
However, giving him massive benefit of the doubt, it's possible there was some mis-communication or he may have forgotten about them. Despite this, he's being a massive twunt for making your kids miss their friends parties, and cancelling last minute.
In future, to avoid misunderstandings about this sort of thing I would exchange information about this sort of stuff in writing (email or text) and agree up front who will do what about the kids attending, before accepting.
The last line of your last post is very telling, though. Does that mean you don't know where your kids are staying when they are in his care? This is very bad form, and shows me the kind of man he is.
As for the upcoming parties - It's hard to advise not knowing your situation. But I wouldb't necessarily discount telling him he can have them for contact after the parties. It should be about the children primarily, and not your ex, and making them miss out becuase he's a twat isn't on.