As a parent I think it is our ultimate responsibility to do what is best for our kids. Sometimes that may be to try and keep the M together but sometimes it may be to let go.
One thing we should never do is to badmouth our spouses regardless of the behavior. The kids have a right to have a healthy relationship with both.
Much as we worry about our kids, they also worry about us and they need to see us coping with these issues and not leaning on them. So be strong for your kids.
Also trying to guilt your WAS into having a change of mind due to the Kids WILL NOT work. They see this as just another tactic regardless of your intention. There is really no benefit in discussing how bad Divorce is for the kids if you are the LBS as that piece is outside your control. We just need to figure out how to minimize the effect on them which is to parent as best you can and cooperate with the WAS in all parenting matters. Show good example and an upbeat attitude.
The M breakdown is about YOU not your kids. The WAS is not leaving the kids they are leaving YOU. If they could replace you and keep the kids they probably would. This is the bit that needs fixing.
I am in the same sitch as everyone else but i think it is important we realize this and don't spend all our times crying for the kids ( especially not in front of them )
I have been a good step-dad to a kid for the past 8 years. His real dad hasn't much time for him and now my WAS is starting to have him raised by baby sitters. Her family and friends have told her this is wrong but she is not doing this to hurt the child she is doing this to get away from me.
Don't ignore the impact on the Kids but put it into perspective and don't let it dominate your thinking. It may be a great reason to try and save the M but it cant be a tactic or the sole reason.