I don't think I would tell them. The affair happened because one of you or both of you were not happy in some way. You have decided you don't want the marriage to continue, so your children although adults now, don't need to know the ins and outs, just that their parents want to end the marriage because they do not want the same things. I would be together on thus because it would be awful, if one child supported the father and one the mother and took sides, you are both their family, love them and hopefully want to share family occasions. In the end this be get better for everyone, if either of you settle with someone else.