I have no legal experience, but am a divorced cp.
My xh has done much the same, he talks bad to the kids about me. He has told them I stole them from him, he tells the kids he and his new wife are the "real" parents. His wife told my 3 year old that my hubby is bad, she came home so scared she wouldn't even look at him. Xh even went as far as calling DFACS on me with false accusations. The report was investigated, then the case dismissed as false. He has become a total scum, and a total idiot- and discussing things or having a mature talk with him is just not possible.
I don't know of anything legally that you can do to make him stop. But, for yourself and your kids, do your utmost to never talk bad about him. My ex does not realize how damaging his comments are to the kids, not in my relationship with them, but his. The kids will see the truth eventually, they will see how wrong and mean he is, and they will see how much nicer you are. It may take a while, but they will see it. They will gain respect for you if you don't talk bad about him, they will continue to lose respect for him. But you have to let them do it when they are ready, you can't force them to do it. My kids are very young (3 and 5) so when they tell me something, I ask them if they think that is really true. Usually they know it isn't, and we just talk about how silly it is for anyone to even say such a thing. Keep the lines of communication wide open between you and your kids- and keep a positive attitude. It will work out for you.
If the harrassement is really bad, and threatening, I think you could get a restraining order. I don't know that much about that kind of thing though...
Sarah and babies