Hi Alex, well since we are so close to Christmas I would not tell the kids till after. One you do not want to wreck their christmas and maybe your wife might have a change of heart during this time. If after Christmas she is still wants a D and involved in her A then that would be the time.
The older ones 13 and 10 you say that their Mom is not happy and she thinks wrongly that is is ok to go out with someone else instead of working on the marriage. You say it is not anything that they did and you are committed to working on making your marriage better. If she has not served you then I would not mention the word D.
The 8 year old -- you need to give watered down version. That maybe you see that Daddy is sad and Mommy is unhappy but that Daddy is trying to make the marriage happy again and is hoping that Mommy will try too. You could say she has a friend that she is talking to instead of Daddy and you want this friendship to end because it is bad for the whole family.
Just some ideas. It is really rough. I do give my D15 credit for doing well during this but it truly took away the last of her childhood innocence. It breaks me up.
Me 53 H 50 D16, D29 M 22 years bomb 7/08 INH - alien pod replaces H 8/08 - OW (direct report), I work there also bomb II - H moves 10/1 expose ow 10/22 D to be final 9-09