Whilst I do not usually believe in slightly sneaky things, I would keep you phone on record when you are there so that exactly what she says is on record, and it can be seen for manipulating intention. Just reading something written cannot give the attitude or tone. I think you and your husband need to talk about what you want to do about this. There are a range of possibilities, from standing back from her and not being involved in any child care, to possibility of speaking to a solicitor who is a specialist in family situations and looking at your choices. So you might end up sending her a solicitors letter , or a recorded delivery one, saying that her defamation of character is something you wont put up with or condone, so that you are calling her bluff and she will see that she cannot use this attitude as and when it suits her. If you let her get away with this she will use it whenever it suits her. and the longer you allow her to blackmail you like this she will then imply that you let her do this as she is telling the truth.
I think that in any case you will be concentrating on keeping a contact with the children, and let them know you are there for them. When the children are older they can come to you without her permission. Good Luck and definitely talk things through with your husband so that you are both on the same page and have a plan.