Hello sir. I hear your concern and it is one that is unfortunately common. There are two things that stand out in your question. First is how and when to tell the children. They will not live in ignorant bliss as long as there is tension between the parents. No matter how hard the two of you hide your feelings, the children will pick up on the underlying problems. I would put the responsibility on your wife to tell them, if she could handle it. She is bringing this upon the family, so let her deal with the children's reactions. That would be my first way of approaching it. If she refused, then it would be time for a family meeting with you and the children and let them know about the changes that are coming. I would not get into specifics about what the mother is doing, but do be honest that their mother no longer wants to be in the relationship. Second, the long-term effect. There is no definitive research to say how things will turn out. However, there are protective factors that can help in the long run. These would include: providing stability for the children, honest communication, one parent who has the children's best interest in mind, social support, and most importantly keeping the children out of the middle of quarrels between the parents. I hope this helps you and good luck sir.
I think you are right that she will probably use the word "we" instead of "I" want to leave the family. By you having the discussion, you have more control over the direction it goes. So, when you tell the kids that mom wants to leave, don't fill in the blanks. Let them go and ask her why. You need to put some responsibility on her for this. I am assuming the children are in school now, so ideally if things needed to happen, it should be done over the summer. Can you and Susan start planning for this? You can tell the children at any time, but the plan may not go into effect until they are out of school. If you and Susan can have reasonable conversations, it would be best for the two of you to start looking at housing situation, custody, visitation, etc. The more you two can agree upon now the less likely lawyers need to be involved. If you guys have a game plan, then when you talk to the children, you would already have a plan to present them with. Unfortunately with so many couples, it is a spur of the moment decision and the family is left picking up the pieces as they go along. What are your thoughts.