I'm 6 years post break up now. There wasn't a script for mine, no OW, just an arse of an XH! We get on fine now, hang out with each other's families at Xmas & kids birthdays etc, all very amicable. He wasn't cut out to be a husband or dad. The latter he can't change, so he does as little as he can get away with!
As for telling the DCs, we tried a few months before the big split, but DS1 who was about 9 at the time, was so distraught, he was shaking and wrote us a lovely letter about how devastated he was and begging us not to split up our family. I felt so bad for him that we did try to make it work, but it wasn't to be.
I think the previous almost-split and the general animosity before the real one meant that nobody was surprised. It was after a holiday (one of the most stressful times right?!) and we got home, had a massive row and I told him to leave. It took a few weeks, which were awful for us all, but once he'd gone and we all settled into our new spaces we were all much calmer and happier.
It must be hard for you, not feeling that there were any problems with your relationship. He must have been covering up a lot of ill feelings very well or he is rewriting the past to suit his narrative. Unless he is a Hollywood actor I'd bet it's the latter!
I agree with many posters on here. Tell your DCs, let them work through their anger and grief, support each other and plan for a happy future. There is never a good time for this news - it's always someone's birthday, Christmas, Fathers' Day etc - we limped along for years like this, just scraping past one occasion to the next! You H is being unfair expecting you to deal with this while putting on a brave face, not being able to show affection or act normally around him. Kids aren't daft, they will know something is up, and often the worry is worse than what is really happening. They will resent you both for keeping them in the dark, lying to them that you were still a family when they could sense that something was wrong. Fuck what your H wants to do, honesty is important for families to build trust.
You will come out the other side of this happy and independent. It can be a great feeling being the head of the house on your own, as daunting as it seems to start with. I'm a happier, funnier and more rounded person than I was with XH and you can be too.