My ex's girlfriend, can I stop her seeing our kids ?

[sheballs]

My kids dad has split up from his girlfriend, he moved in with her 6 months ago and the kids have gone and stayed with them every other weekend (2 night stay) for the past 6 months.
Now they have split he has told me about there relationship she's been violent to him quit a few times and told him how much she hated me, by body shaming me on social media and telling him she wants to get the kids taken off of me.

Now a couple weeks/months down the line he's telling me he wants to get back with her and thinking about moving back in with her.  After everything he's told me about her I don't trust her around my kids and what if she flips and does something to them. I would never stop there dad from seeing them but I'm scared and don't know what to do. They live 2 hours away from us. 

Does anyone now if I can legally stop her from seeing them as I don't have evidence of what she's done to there dad only his word, but I think he would deny it if it went to court or something like that.
What can I do pls give me some advice

obviously i do not know if you have a court contact order with your ex, but you need a serious talk with him about your concerns and that you do not want your children to be around her if she is known to be violent.

I would stop the contact based on the violence and tell him that he needs to come and see the children for the day only and there won’t be any overnights.
If he disagrees he can take you to court and you can tell them what he’s told you; try to think and write down if he told you any specifics of when it happened and let the court decide. During that process it may come out that she has a previous violent criminal record or that there have been police call outs. It may also make your ex think twice about getting back with her.
Don’t think that you just have to let them go there because the courts might not believe you; it could be really dangerous for your children and the courts will see that you’ve let them go before so why would you stop it now without a really good reason.

Be very careful with issuing ultimatums.

I did the same with my ex and he decided to just stop seeing DD.

Could you email or text him about it to try and get him to confirm what he’s said in writing? That way you have evidence it it does go to court.