Split with exDH couple of years ago, long story but he went off with younger woman from work, reasonably amicable (rather glad to see the back of him in the end!) and he has kept good contact with our two sons aged 7 and 9.
He usually has them on a Friday night and when he picked them up this week he said they were going to stay round her flat.
Now I don't know this girl very well, except that she has mental health problems - manic depression - which I know shouldn't really be an issue. The few times I met her she seemed ok but I have heard stories about her that makes it sound like she lives quite a chaotic life. So far she hasn't been a huge part of my kids life, she sometimes goes on days out with them but usually exDH had my kids stay with just him at his home (lives with his dad). As far as I know she has always been nice to my kids and they seem to like her, which is fine. ExDH has had alcohol and drug problems in the past but seems to be at a good place at the moment.
BUT I feel quite uncomfortable about them staying at hers. I don't know why it should matter so much to me. I don't like having to share my kids at the weekend anyway and now I feel thoroughly fed up that they are staying with a woman who I'm not sure about.