I am so sorry that you have to deal with yet another betrayal on top of everything else. Please don't fall for any of that junk your H and OW try to throw at you. He made his decision on involvement in the OCs lives now, as well as his decision to not have involvement in the beginning. You gave him ample opportunity to express his views, which he chose not to do. Don't let him give you the guilts now, and try to blame his latest escapade on you.
Some men always try to justify away their sorry behavior any way they can.
I hope he and his wretched OW get what they deserve. Please stay strong for them, if not just for yourself. You are definately in my prayers. Please let us hear from you. The Lessons Children Learn.