Ex of 2 years sees the children (10 & 8) one night a week. We don't speak apart from emails to deal with issues and contact times with the children.
As far as i was aware he has a long term partner of 2 years,(who i believe he left me for). Anyway, kids arrive home to me clearly upset that in the middle of the night they heard a woman chatting and laughing and listening to music with their daddy. In the morning they went in to say good morning and the woman hid her head under the covers. They helped themselves to breakfast and went on the xbox (sat on the couch next to her white fur coat which 'stunk' of strong perfume 
)while he spent the morning in bed with this woman (who wasn't the girlfriend) (dad of the year or what?). At lunchtime the 3 of them went to the a shops without her and she wasn't at his flat when they got back. DD said she heard him on the phone later saying " i know baby, less than 24 hours ago we didn't even know eachother. Its amazing) (such an astute girl my dd
)
Anyway kids upset DD said it was really creepy and she can't get the situation out of her head. I've said if she needs a break next weekend and stay at home then thats fine but if she wants to visit daddy as normal then thats fine too. she said she wants to stay home. So now i need to write an email to him about next weekend.
He has 6 nights a week to shag around but I'm so shocked he did this on his one contact night with the kids, with them in their beds in the next room. Ewwww. just Ewww. My poor kids having such a dad. Not sure how I can deal with this. He's narcissistic with psychopathic tendancies (what a catch he was!) so no point getting preachy with him but I do want to protect my kids, obviously. Whats best way to deal with this issue?