mummyme - seriously, I think if your DC are all in their teens, leave it up to them now. Have a frank talk with them about it though - tell them that you think it was very bad behaviour on the part of their dad to lie to them like that (you CAN say that, it's fine to hate the behaviour, remember) and that you can't understand why he wouldn't want them there, as they are such lovely DC. Make sure that you let them know that it is in no way their fault that they were excluded. I would imagine they have a pretty clear idea who is at fault here anyway; they seem to be fairly well clued-up as to what they're up against, but it will help them if you support them by telling them that they are wonderful and should have been included.
As far as them seeing him again - well I doubt he'll chase them for contact after this, but if he does, just hand it over to your DC to deal with. Tell them that they may see him if they want to (and he remembers to turn up) but you aren't going to push it either way.
As I said, if it were me, this would be the last straw - but I know that sometimes children will go to any lengths to try and stay on the errant parent's good side, to try and elicit any sign of love. It's not good for them, but they have to be allowed to try if they want to, with the knowledge that you will support them whatever (while still pointing out to them that him missing out on seeing them is firmly HIS loss).
Horrible situation but I think you would be doing them more of a disservice to push them into seeing him than letting them choose.