Sorry, just have a minute... So sorry this happened. Sadly, it is not unusual or surprising in these circumstances. I hope you will talk to some friends or family to get support.
Now, he can do those things because you choose to do them. You are not in a committed monogomous R and you know this. If you were, you could talk about OW and your fears and ask for reassurance. The topic is taboo because you are not in a committed monogomous R. The thing to ask yourself is whether you are happy have sex with someone outside such a context. It seems not. So why are you continuing to do so? Why do you want a level of emotional involvement and dependency that you have maintained in such an R?
Look, boundaries are YOUR boundaries. Not boundaries about what he can or can't do. You can't control him. Unless you are going to legally prevent him from seeing your children by himself, you cannot prevent him from seeing them with OW. You cannot force him to tell you the truth. PEOPLE LIE ABOUT A's and the stuff related to A's ALL THE TIME.
What can you do?
1) Do not stay silent with your children any longer. You cannot trust him to share info with them appropriately. You can choose to share info with them. Invite him to the discussion or not.
2) Make sure your kids know that they do NOT have to keep daddy's secrets.
3) Don't sleep with him, you resent him for your choice here.
4) Decide what R with him works for YOU in this context and act accordingly. Are you content with being one of his multiple girlfriend's and sexual partners? Are you content with being a platonic friend? Are you content with being merely a polite co-parent? To be honest, I don't see how the first two would be good for you. You want more than the first and he is not treating you like a friend. But, YOU need to decide what YOUR boundary here is in terms of physical and emotional intimacy and YOU need to maintain it.
5) Decide what level of trust you are willing to risk with him on what issues. As far as your R, the A, the kids, etc..., I'm not sure why you would expect him to be honest with you now. Until you can have frank open discussions about such things and until he is willing to work to earn your trust, my guess would be that he is not ready for a committed monogonous R. In general, I don't think anyone dating more than one person is entirely candid with the other dates.
He is NOT doing these things TO YOU TO HURT YOU. He is in a very different R than you continue to be with him. Step away -- you don't seem to want the R he is in with you. You want him to have the feelings you want him to have and the level of commitment you want him to have. He is not obligated to do either. Would you want him to act out of obligation in any case?
Get a job. Take care of the child care arrangements. Quit giving up your life to support his -- you are no longer partners and it is not reasonable of him to expect this from you.
Plus, write really nasty things about them and burn them up. He is being a total creep. Then, go out and find a friend and a bottle of wine.
BTW, had you ever expected A's or inappropriate behavior before the current OW? I hate to say it, but to me he seems like a man that has not been used to monogomy for sometime....
