Does he abuse them or neglect them in any way or do you just want to leave and feel your kids shouldn't be near him so you don't have to deal with him?
If it is the first one that he's not a real father to them then counseling is probably the best thing for them and you not talking about him and reminding them about him is the best thing.
If it's the second part, I would say that you'd be doing your children more harm than good. Your children shouldn't have to be without their father if he's good to them and loves them just because you're angry or hurt. If that's the case then I'm sorry to say that you'll just have to put your own feelings aside so that your kids aren't denied the love and attention of a decent Dad.
If you are just simply asking how to help children cope with separation, I say that if you and their father can work together to be sure the kids know it's not their fault and that they are still the most important thing to both of you then it should make the adjustment easier on them. Keep your issues with each other away from the kids as well.
I'm sorry I couldn't be more help to you, there are too many Ifs to be able to give more solid advice.
how old are they? how much is he involved in their lives now? Kids adapt quickly and as long as he still sees them regularly they should be ok. They just need to know they are still loved and feel safe and secure. Its not easy, but kids adjust.
good luck