Hello, I am so sorry you and your family are in this situation. I can speak from both sides of this. My Dad was not a part of my life as a child and unfortunately it has repeated with my children's father as well. It is especially painful for me because I know what they are feeling.
I would recommend counseling for them if you can afford it at all. I think that may have helped me because my Mom did not really talk to me about it and I saw her going through alot of pain when he left.
Also try to find male role models for them. Grandpa? Uncles? Family friends? I think its important for them to see whole family units with a caring father. For them to see that all men are not like that and they can expect more from a man and your son can be a better man. I never got to see a whole family and how that even works together, so it was very hard for me when I had my own family.
I now have a relationship with my father (I'm 31). We talked about why he was not a part of my (and brother's)life when we were growing up and it seems to have been a combination of him moving on with his new family, having obligations with them and also it being uncomfortable for him to see my mother knowing how he had done her wrong and re-live that feeling everytime he saw her.
I don't know the specifics of your situation but I think you have hope because
1. he was a caring father for them before- so this not his usual behavior
2. you are willing to set aside your feelings for the sake of your kids
How about have your daughter write him a letter telling him how this is making her feel? If not a letter to him, get her a journal so she can write/draw about how she is feeling. It is good for her to recognize her emotions and release them. Also it may help for him to hear it from her and not you.
Maybe you guys can work something out where he picks them up straight from school for the weekend and drops them off on Monday or at a friends house- some arrangement so he does not have to interact with you. (generally men do not handle with their emotions well)
If you can't afford counseling there must be books on this at the library, in fact I think I am going to check that out also ;>
When in doubt pray and then keep an open heart and mind to see the best solution!
A.
Mom to 4 - 8b,5b,5b,3g