My son is having some respect issues with his mother. She had an affair three years ago at her work which is an elementary school. She worked in the kitchen and had an affair with one of the janitors. The principal at the school caught them in the act at school. The principal went to the same church as my family. He later told me about it. I have never confronted my wife about this but have seen footage from the school security cameras. The janitors son told my son about all of this last fall after his Dad told him what had happened. My wife now suffers from rheumatoid arthritis and is no longer to help with caring for our home. My son gets very frustrated when I am busy caring for our home and am unable to do many of the things we used to do as a family. This causes him to act in a very disrespectful way to his mother. I've called him on it several times now. I'm not sure how to get him to grasp that our life is going to be very different at home because of my wife's physical limitations.
Generally, we know that a person must confess to be forgiven, and that is a key way to be forgiven in relationships also, to the people who feel personally hurt by the action. The person confessing shows contrition -- one might say "I did something very wrong, as you know, and I have confessed this to God. I know He will forgive me. I am becoming a new person and will not want to ever do it again. Please forgive me."
Depending on age, if at least 7, and more likely 8 or older, children could learn to forgive directly from the gospel passages about it. Not the more adult like passage parable of the Unforgiving Debtor, which relies on a more adult understanding of our own guilt of sin in order to understand the parable, but instead the more learning over time way could help. If a person really believes in Jesus Christ risen, then they can sincerely pray the Lord's Prayer He gave us in Matthew chapter 6, where we pray "And forgive us our sins as we forgive those who sin against us." And the effect of praying this prayer daily or weekly over time can instill the message that we are to forgive others.
We could even eventually read to them, if about age 9 or older Christ's more directly strong wording on forgiving:
14 "For if you forgive other people when they sin against you, your heavenly Father will also forgive you.
15 But if you do not forgive others their sins, your Father will not forgive your sins."
This is very bracing, and it's fine to just read it and leave it at that, and let them have time to think on it. It's a lesson for all of us, even those much older, that still need to forgive someone, even when they have not confessed or repented. We must forgive them for our own sakes.