What should I tell my children about the affair?
Note: The Affair Healing for Betrayed Partners Course has a section focusing on this issue (Part 7: Who Else Should Know?), so use that resource if you want more help.
Be honest with your children, but what you tell them depends on their ages and how much detail they already know. In all cases, the information is best communicated by both parents, with the involved spouse taking the lead.
Even if you choose not to tell your children now, be open to discussing this at any point in their lives when they struggle with significant failure. Your authenticity during those moments will be much more beneficial than trying to protect their impression of you.
Children living outside the home: If they don’t know, I encourage you to wait until the healing in your marriage is well established. They may not need to know. But if they ask specific questions, do not be dishonest.
Teenagers or adult children living in the home: 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 have any awareness of the affair, admit that you got too close to someone else for a while, but 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 will 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 any conflict they may witness. 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.