what and when to tell children

My dday was Jan 1st. Fear, scared, angry and numb are just a few of the adjectives that describe the utter pain and sorrow I feel. We have 2 children, ages 18 and 22. Our18 yr old will be going off to college in the fall. She knows something is wrong as I have been weepy. We both sat her down and told her we were going to counseling to work on our marriage. Is it necessary for children to know about their father being unfaithful? I think it would be devastating to her to know that her dad has been unfaithful for 2 yrs