You know, no doubt there are bad people in the world who we should keep away from our children.
But, until you let go of your old narrative and quit being a victim, you can't really get an objective picture of whether and how GF might be a threat to your kids.
You are too invested in her being evil and thus being the reason your M failed. It makes it about YOU.
Getting to a more objective place would actually help you more when it comes to the kids.
Has she actually done anything that is mean/evil/harmful to the kids?