I am not as far down the line with this as you are Newstart, but I have similar concerns. My older 2 are old enough and stable enough to make their own decisions, and I have made this clear, I have told them that I am not going to ask them to take sides, or think any less of them if they want to meet her or have a relationship, it's going to hurt like a bitch but I will respect it. Although I am being "reasonable" I can't use her name and refer to her as "this girl" or "this woman" I am only human and the situation is all very raw.
I have also made it clear that my youngest who has ASD will not be meeting the OW until I am sure that it is a stable relationship, and I am looking at at least a year, I don't want him forming an attachment that isn't always going to be there. Also because these 2 people have the morals of an alleycat, neither one of them are acting like people that I would allow round my kids, and actually neither would my husbad previously.
I hold my husband responsible for his behaviour, he went out chasing this and didn't do the adult thing, he should have left and we could have had a different relationship with him and also with her, and he has stolen that from all of us, including her. I don't hold her equally responsible because this girl doesn't know me, but she knew we had kids, and she allowed herself to believe the lies and BS my husband was feeding her, so frankly that is not the kind of woman I feel is a great role model for my kids.
I am just hoping for my kids sake that he steps up now and starts being the Dad he hasn't been for a long time. I have told him that his relationship with his kids is now his responsibility, I am not going to stop being reasonable, and I will do all I can to faciltate whatever access he wants for everyones sake, including his. If he has any respect for his kids at all, he will leave a decent amount of time before he expects to introduce her to anyone else. I am trying to separate my issues with them from my issues with the kids, his behaviours are going to continue to hurt me, but I can't let my BS cause conflict for the kids, it's a terrible position for them to be in.
Not saying you are, that's just me, if he cocks up continually, the kids will eventually be old enough to make their own minds up, and as long as they have someone that supports them and allows them to express what they are thinking, they will be grand.
I do not understand what makes any parent behave like this when there are kids involved, if you love your kids they should always come before your libido!!