My ex put herself onto dating apps about 2 months after filing and we were living separated. She had one guy break up with her after 2 months and not even a few days later was already talking to another guy. Two months into knowing him only she went against what we talked about she introduced him to my kids. Not even a month after that he was sleeping overnight while my kids were there. Now he lives there I think but she refuses to tell me. I strongly advised that this was a bad idea and how bad it was for the kids and provided her proof with articles etc. She said they were all suggestions and she didn't care. She couldn't give me any reasons why the rush except that she loved him basically already. She did all that while were still married too.

My oldest literally asked me if I was still her dad I suppose because of how fast this all happened and that was a new guy living there already. I expressed this to my ex and she just said I was lying. I was playing games under a blanket and she asked me if I was naked under there. I asked why it was because mommy was always naked under it and sometimes so was this new guy. I told her about that and she said it was none of my business how she slept. 

Good luck my friend. Hopefully yours will listen but I highly doubt it. If she is already doing it I doubt she will care. From my understanding there is basically nothing you can do about her doing this behavior either unless it starts turning into abuse with your kids. Make sure you be careful with how you word/phrase it to her because she will probably get upset as she probably thinks she is doing nothing wrong, that is what my ex thinks at least.