Ahh.. it has begun. The unraveling. I'd give their relationship another four years, tops.
Conflict with the BS often binds a WS and OP in a common cause. When the BS removes himself/herself from conflict and interaction- it's one reason it's best to go No Contact other than kids' schedules - then the WS and OP must get to know each other.
In an affair, deciding which kind of sex to have and where to sneak away to meet, and getting intensely excited in the anticipation, is about all there is to the depth of some relationships, and when that sexual energy beings to wear off when there is nothing to anticipate but same-old sex at home, the illicit couple needs a binding agent.
It seems that your ex and his OP have already hit the stage where they must recreate the "monster" and fight it together to keep from falling apart. Battling you is all that they have in common.
This is why an angry and sad BS who says nothing and stays distant is preferable to a sad and angry BS who finds ways to lash out toward the illicit couple.
Tell your kids to laugh it off and do the roll eyes at them the next time they start up this stuff.
Her kids likely want to think of their mom as a good person and you as a bad person, and not think of their mom as a sexually available cheater who screwed a married man and hurt a nice woman. I feel sorry for all the kids in this situation. But it seems her kids need to demonize you, just as their mom and your ex, to justify all of this to themselves.
They may keep doing this to you for years, so I'd suggest getting one of those calendars that automatically emails reminders to all, and jot down all school events, plays etc., so they can't "blame" you for not still holding the social secretary part of the wife job for him.
[This message edited by Heavy Sigh at 10:04 AM, May 27th (Monday)]