What should I tell my kids?
I always encourage honesty. Trying to protect yourself, or them, by lying often provides temporary relief at the cost of future consequences. Learning that “the ends justify the means” is not a lesson that contributes to secure trust in a relationship. It is better to say, “I’m not ready to talk about that now,” than to provide a false answer to their questions.
What you tell them depends on their ages and how much detail they already know.
All Children: If you decide not to tell your children now, remain open to a time in the future (perhaps many years from now) when one of them may make a bad choice that results in feelings of shame and failure. At that moment, one of the best gifts you can offer them is a willingness to be authentic and vulnerable by telling your story.
A book I often recommend to unfaithful partners (especially men) is Scary Close by Donald Miller. It's a book about our struggle to be intimate and vulnerable in relationships. In Chapter 12, Great Parents Do This Well, Donald writes about his conversations with Paul Young, author of The Shack. Paul chose to be very open with his children regarding his affair, and that family's perspective provides a helpful insight when considering what to tell or not to tell.
Adult Children: In most cases, if your adult children don’t know about the infidelity and you and your spouse are exploring the possibility of staying together, I encourage you NOT to tell them. Not at first, anyway. Wait until you’ve experienced more healing. When emotions have stabilized, and you both have made progress toward whatever choice has been made regarding your future together, then decide if they need to know.
An honest explanation should be provided if you are moving toward separation or divorce. You and your partner should agree on the exact message. If you cannot agree (which often means the injured partner wants them to know about your infidelity), you are likely better off telling them. Your children, however, do not need to know too many details. Use broad brush strokes to paint an honest picture, but avoid the details. Hopefully, for your children’s sake, your partner will be willing to cooperate with this approach.
Adolescent Children: If your children are teenagers, they likely already know or, at least, have guessed what is happening. If so, don't lie about it. Without getting into details, you should admit what happened but assure them that it is over and that you are taking steps to fix your marriage. (Caution: Don't tell them this if it isn't true. Telling lies to assure and comfort them may buy some temporary peace, but it ultimately does more damage to their trust in you.)
If they have questions, answer them. Keep the lines of communication open with your children, and don't be afraid to check in with them periodically to provide assurance and answer any new questions they might have.
Ages 8-12: Provide fewer details, but if they are aware of the affair, you should admit that you got too close to someone else for a while but that it's over now. Take responsibility. Give them plenty of reassurance with your words and your touch. Let them ask questions and provide answers that are general but honest.
Under age 8: Your child is likely too young to understand anything about an affair, but they'll undoubtedly have some sense of the conflict in your marriage. Address the conflict. Let them know the two of you are having some problems that you are working on. Assure them that they have no responsibility for the conflict. Children that age tend to view the world as though they are at the center of it and will assume that your problems are their fault.