The Bible speaks about the law in many places, but particularly through the Ten Commandments. Children must understand that sin is not just an outward act; its roots run deep into the human heart. Because the Ten Commandments are a summary of the law of God, they can usually be used to illustrate our children's sin problem.
For instance, if I witness my son teasing his sister, I can show him how his behavior violates the sixth commandment, "Thou shalt not kill" (Ex. 20:13). I can say: "Son, when you tease your sister in a hurtful manner, you are breaking the sixth commandment. And God hates all sin. You think this cruel teasing is funny, but God takes it seriously, as a form of hatred, which is the very root of murder. You are breaking His commandment, and that is what sin is."
We must explain to our children how the Holy Spirit uses the law to drive us to the Lord Jesus Christ. We must teach them that the law is like a mirror the Spirit holds in front of us to show us who we are. In our natural state, our understanding is darkened, even when we look into the mirror. The mirror is there, but we cannot see into it very well. We see a few flitting shadows, as it were, and we may have a few convictions about particular sins. But when we are born again, it is as if the Holy Spirit turns a bright light on us, and we see ourselves for what we truly are. He turns on the light of the Word shining through the law. This light shines into our hearts, and we see the sin that dwells there. Then we truly recognize our desperate need for the Lord Jesus Christ, our Savior and Redeemer.
This excerpt is taken from Parenting By God's Promises by Joel Beeke.