Research tells us that the best outcome for children whose parents separate is for them to continue to have regular contact with both of their parents.
I wonder how involved your children's dad was with your children when you were married? Often parents who are unreliable after separation were never very hands-on anyway, even when they were all under the same roof.
That parent might love their children, but were never dependable. So, possibly, not having Dad very involved won't actually not be such a big change for your children when they eventually get used to the idea of the separation?
Do the children feel rejected, or are you angry about his lack of involvement? Or is it a bit of both?