rickman in years to come, your kids will be able to see that their father put his dislike and resentment of their mother before his love for them. (You could actually point out to him that that's what he's doing, putting his need to get one over on you above his children's welfare. What a crap thing for a parent to do.)
I think it's quite important to remember that children get much of their sense of self-esteem and self-worth from their parents (as long as both parents are still involved - I don't think that's so true when one party is totally absent). Having a consistently negative message about one parent is extremely damaging for a child imo.
But then, your kids haven't had that, have they? You've slipped up once, totally understandably, and in future years your kids will understand that as well, and marvel that you didn't slip up more often. FGS, there are all sorts of situations where we make parenting mistakes, and our kids forgive us if the majority of our parenting is good enough. This is just another one of those situations.
I think it's helpful to remember the old catholic dictum "hate the sin, love the sinner". If someone behaves badly, it ought to be pointed out that their behaviour is bad (how else will children understand that some behaviour is unacceptable?) but not necessarily that the person is bad. Just as we try not to tell our children that they are naughty, but we let them know that the behaviour is naughty, the distinction is one which kids do understand and so the fact that Daddy's behaviour may be bad, doesn't make Daddy bad. It just depends on how bad Daddy's behaviour is, whether you choose to comment on it. For example, if Daddy is a murderer, I can't see how he can't be criticised if you want to impress on kids that that's wrong; but as your xp is just a shitty person, it's far more difficult to know what can be commented on or not. I personally find his lack of concern for his children's welfare morally abhorrent; and at some stage when they're old enough for their own sense of identity and self-worth not to be threatened by that, they need to know that that behaviour is not right (otherwise what's to stop them repeating it?). But while they're young and forming their own idea of self, I think you should try and steer clear of criticising him (but I know you already do).
Sorry, very long!