You're the dad. You love your children beyond comprehension. Your kids know how to love through you. You are their heart, their anchor, their safe place. Their mom's actions reek of abandonment. And you're left holding the cards. And it's no longer time to "protect the mother image".
Actions bring their own consequences. Her actions dramatically affect your children. You're at the pulse point.
What is the best thing for your kids?
I struggled through my childhood because of the actions of my father. As a child I regularly choked myself with my jump rope, would leap off high places, pick fights with guys much bigger and stronger than me. As an adolescent I'd stretch out in the middle of the road, hoping to be run over. My mom saved me from climbing out a window (with a three story drop) when I was having a living dream. Apparently I'd been calling out, struggling to open the window and storm window. My mother got in just in time to pull me back from plummeting out. I don't remember a thing, but it traumatized her.
The difference is.. you're right there. You know the right thing to do for your children. How to keep them safe, the correct tools to use, when to bring in professionals, how to address it and support them through this process.
The safe, loving parent, the dad. And being a Dad is more than enough because you and your kids are blessed to have each other.