Any man who treats a wife better based on her putting on a mask of happiness is one I would have kicked to the curb a long time ago. After tightly sealing the lawn n leaf bag.
Your husband is failing you and your and his children IMO. He is bearing down to pursue a footloose existence centered on his whims and desires, and he is using psychological pressure on you, punching your insecurity buttons.
You need support in your corner to help you clarify this situation. I'm glad you are seeking it, and proud of you. You deserve better than a loveless marriage to someone who manifests selfish behaviors and puts in so little at the human level.
As GalaGirl said, he signed up for better or worse, etc., to be your partner. He implanted babies in you; he is their father; you are his wife. This is not the time for him to be chasing a stiffie and calling it poly and evading his responsibilities. He's not getting YOUR one single relationship right; how is he going to qualify to do others?
You need to expect, and demand, MORE of him. Not make excuses for his behavior. "It's hard to see...." Not it's not, hon. It's clear as the noses on Mt. Rushmore.
Having said all this rather bluntly, please know that I empathize with you and your situation, send you warmth and caring thoughts, and truly hope you can use everyone's input to hearten yourself. Something in you is looking for a better deal for you and your children; something in you is pushing you to change for the better. That is a good thing. But part of it is going to mean you not excusing things that you may have in the past!
Oh, one last thing? If your husband cannot get the kids tucked in in 45 minutes, and it takes hours, that's a measure in my estimate of how absent he has been. First, because he doesn't know what he's doing (by now he should have this down) and second, because the kids are apparently so starved for his attention, they act out.