It's generally known that divorce damages children. I spent months researching when it became obvious my wife was having an affair and was going to dissolve our family. I pleaded with her to stay until at least our boys were off to college (four years away). She decided against it, left, and now our boys are hurt, angry and confused. I've gone to great lengths to try to protect my wife from their angst, but I cannot control their feelings. She blames me for their occasional harsh words towards her, has accused me of "poisoning" them against her, and has shared that with her family. Now I'm hated for something I haven't done. I refuse to speak poorly to the boys no matter what she's done. As you have pointed out many times, our boys need both parents – but it hurts to be hated. What is good ex-etiquette?
You should know it's really not your job to protect your wife from your children's angst. It's your job to protect your children and reinforce that they are loved by both parents.
You don't have to make excuses for their mother. She can communicate directly with the children; just do your best not to further undermine her now. Here's why: Your kids are looking for stability and someone they can trust. If you start badmouthing their mother, or even agreeing with them when they say derogatory things, it will further undermine their security.
The way kids in this position have explained it to me, it's not that she left Dad, but that she left at all. A kid's logic tells them that the parent who left loved someone else more than they loved them. "Maybe if I was a better kid, she would have loved me enough to stay." They are not only hurting because their mother left, but they are questioning their own self-worth.