You have a specific process for adults to follow if they want to forgive. Do you have a similar process for children?
Children usually do not have the cognitive maturity to deal with 20 units of a rather complex process. I recommend starting smaller with children by introducing them to the concept of inherent worth: We all have built-in, unearned value. As a child begins to see this in loved ones, you can start to generalize this by asking: Do you think that people who are unfair to you and hurt you have built-in worth? Why or why not?