Can you remind her of all of the positives of not being together?
Buy her a copy of Why Does He Do That?
Point her towards the online version of the Freedom Programme?
This is something I read today, bear with me, I think it's helpful.
She is in all likelihood operating under a false schema (set of beliefs and thoughts) that this is the best course of action. Wikipedia on schemata:
New information that falls within an individual's schema is easily remembered and incorporated into their worldview. However, when new information is perceived that does not fit a schema, many things can happen. The most common reaction is to simply ignore or quickly forget the new information.[16] This can happen on a deep level— frequently an individual does not become conscious of or even perceive the new information. People may also interpret the new information in a way that minimizes how much they must change their schemata. For example, Bob thinks that chickens don't lay eggs. He then sees a chicken laying an egg. Instead of changing the part of his schema that says 'chickens don't lay eggs', he is likely to adopt the belief that the animal in question that he has just seen laying an egg is not a real chicken. This is an example of 'disconfirmation bias', the tendency to set higher standards for evidence that contradicts one's expectations.[17] However, when the new information cannot be ignored, existing schemata must be changed or new schemata must be created (accommodation).[18]
When you tell her "He is dangerous, this is a bad idea, your children are in danger" she is likely to do one of two things: She will ignore, forget or minimise your concerns, either ignoring or forgetting the conversation ever happened, or deciding that you are mistaken, you think he's worse than he is, you don't understand the real him etc. She may also possibly decide you are a threat and reduce contact because you threaten her worldview and, therefore, her safety (I know how backwards this is - it is backwards - but it's a survival thing. Just unfortunate that the survival instincts point/lead to actions which are the opposite of survival!)
I can totally understand your personal reluctance to keep quiet, but do know that it's not a personal thing against you that she's doing - it's just the headspace she's in right now will not let any other views in. There are only really two ways to change that headspace - either to keep planting seeds which will make her look again at the evidence and see it in a different light, or to confront her very forcefully with something she actually can't ignore. This second option, you probably don't have the power to do - because you can tell her until you are blue in the face that she or the children are in danger or that Social Services will remove the children and she will keep ignoring, minimising or changing her view of you almost infinitely before she challenges her own view of him. Similarly in some cases even Social Services involvement is not enough of a confrontation because victims see the chink of "Perhaps they will see he's not so bad after all" and that keeps confirming their schema. It would have to be physical and immediate - a direct threat to her life or the life of her children - to make her immediately change her mind. Of course you can't do that.
She is probably much more able to accept smaller wedge shaped pieces of information that lead her to explore stuff for herself - ESPECIALLY if she's wavering on a decision - than to be told that such a course of action is dangerous - that's too harshly opposed to what she's believing at the moment. If you've already said that then don't worry, it might sink in somewhere, but keep feeding small nuggets of stuff about healthy relationships, about abusive relationships, about self esteem, etc, because it just might be that something clicks. And make sure she knows that YOU care about HER and SHE matters, because she might not be hearing that from a lot of sources right now.