The best thing you can do is tell the kids that their daddy loves them very much but can't see them right now because he doesn't feel very good.  Tell them of all the good times they had with their dad before, and answer anything they ask you with as much honesty as you can.  Avoid putting him down at all, they will remember when they are older who was there for them but if you put them down, and he tells them that you have turned them against him and that's why he never saw them, they are less likely to believe it and more likely to be able to build a bond without blaming you unduly for anything.  Keep trying to get him in contact with them, but don't force the issue until he is ready for it.  Just be supportive and talk about your ex with them as much or as little as they want.

Is there any way you, a mediator or your lawyer can talk to your ex about your daughter & how she is feeling?  Explain to him that it's not punishing you, but your kids?

Is there a trusted adult who can spend quality time with your daughter - not as a replacement father, but as a close friends, someone she can talk with and have fun with?

Just a thought.  And hugest hugs to you & your daughter.