My ex got his new gf pregnant when dd was 10 days old, after leaving me when I told him I was pregnant, because he wasn't ready for children. It does hurt, OP, and I understand your anger.
I remember ex ringing me a few days after his new child was born demanding to see dd, despite hardly seeing her throughout his gf's pregnancy. He was so adament that dd needed to meet her new sister. She was 9 months old for goodness sake and had no idea who he was, never mind the new baby!
Anyway, the reason I am saying this is because I remember very well what it was like to be in your shoes. However... 5 years have passed since then, and with time and patience, we have all moved on. I am married now and have another child. Ex is married to his gf and they have another child together too. My dd loves all of her siblings so much, and loves spending time with them. The hurt has gone now that I felt about ex, and it's a huge relief.
I must admit, I don't like that they sign cards from her as Mummy, and dd says they always tell her to call the step mum Mummy, but dd doesn't want to.
I wouldn't worry about your dc meeting the new woman yet, there's plenty of time. But they should, of course, still be seeing their dad on a regular basis.
Good luck 