It is interesting that the lessons I teach to my 4/5 Primary class are the same as those taught to everyone else. True the ideas are more simple with stories and such, but forgiveness is taught on all levels.
"And be ye kind one to another, tenderhearted, forgiving one another, even as God for Christ’s sake hath forgiven you." Ephesians 4:32
Previously, I didn't quite get WHY I needed to forgive, except that it was a commandment. Without forgiveness my own repentance was incomplete. "For if ye forgive men their trespasses, your Heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses." Matthew 6:14-15
Even that doesn't tell the whole story. How could being reconciled to my brother (Matt 5:24) or loving my enemy (Luke 6:27) benefit me in any way? I thought forgiveness was another sacrifice to demonstrate obedience, which I was willing to do no matter how hard it was. The main reward of forgiveness is hidden in this verse. "And all thy children shall be taught of the Lord; and great shall be the peace of thy children," Isaiah 54:13. If we learn Jesus' ways and follow them, including forgiving others, we will find great peace.
Forgiving others may or may not benefit the forgiven but will always benefit forgiver. Our negative emotions, such as anger, revenge, bitterness, and so forth, melt away, when forgiveness takes place. Like most commandments, forgiveness gives peace. Without this understanding, I was slow to forgive. Now, I know holding a grudge doesn't offer as much reward as does letting it go. I'm not sure this idea will entirely make sense to a 4/5-year-old. Nevertheless, as they are taught this over and over through the years, likely, eventually epiphany will occur. I was well into middle age before I got it.