Hi, name changed as details are very identifying but necessary for context.
Separated from H for some time, but not quite 2 years yet. Amicable, previous very good coparenting relationship. He got together with a woman not long after we split who was very nice, kind to my children, had no issues with her, sat her in the family pew at my DF's funeral as she was supporting H. Few things that grated but decided not to make a deal out of them and we all rubbed along nicely. Sadly they split just after Xmas.
H is now with a new girl, A. Been seeing her 3 months or so. One night about six weeks in to the relationship he was panicking, whatsapping me for advice as she had got very drunk at his house, ended it with him because she 'couldn't deal' with H and I having an amicable relationship. She went on to 'self harm' (lightly scratched her arm with kitchen scissors) then got in her car and started the engine. At this point he told me to move my car away from my house as she knew where I lived (same small village as H, just down the hill) and he was concerned she would deliberately drive into my car due to her 'major resentment issues'. She drove off (not damaging my car) and he called the police as she'd necked nearly a bottle of gin. She returned after a drunken lap if the village, police attended later and took no action but recorded it as a 'concern for safety'.
To my horror he took her back after this escapade. She has blocked me on Facebook (we weren't friends but I am with H) because it is 'too traumatic' to see me commenting on his posts. By his admission, she has a half-hour paddy if my name is so much as mentioned. H spent DS' birthday with us and she had a tantrum about it.
Far worse is the change in his behaviour, clearly driven by her. We have gone from amicable exes to him being quite abusive to me in messages, telling me I am controlling, manipulative, awful, because she says I am. She has virtually moved into his house and only goes home when he has the children. She is planning lots of renovations which apparently they will jointly pay for. 
She has not yet met the children. After 'the incident' he said he would leave it a long time, let me meet her first, etc. Now apparently they will be meeting her soon, she has already started buying them presents which he gives to them. I have serious concerns about her mental stability, but also the impact she could have on the kids' emotional wellbeing. They are going to talk about me - I'm their mum! - and I'm worried about what she will say/do when that happens and how that will impact on them.
I am pleading with him to hold off. I have now instigated divorce proceedings - we were going to wait 2 years for separation grounds but various factors make me want to hurry this along due to her influence on him - and when we go for mediation I want it recorded that they are not to meet her without my permission. He has form for making less than stellar safeguarding decisions around people he kowtows to and he kowtows to her. My 10 year old in particular is super sensitive and will pick up on negativity and be very upset/worried about it as she idolises me.
AIBU in my outlook? And does anyone have experience of requesting similar conditions during mediation/child arrangements in divorce?