Children see things in black and white. This means they see the difference between right and wrong. However, they focus on external matters, so we must be concerned to reach their hearts, which have been blackened by sin. Specifically, we must be concerned to awaken our children's consciences and strengthen the operations of this God-given faculty, the remnants of "the work of the law written in their hearts" (Rom. 2:15), sometimes called "the light of nature" (Westminster Confession of Faith, 1.1).
As parents, we need to remember that the problem of sin in the heart is the most important thing young ones need to learn. That message should be worded in very simple terms for young children.
For example, a four-year-old child might see a group of men out on a golf course on Sunday and say to you, "Daddy, look at those bad men; they are playing golf on Sunday instead of going to church." Such a comment shows how children tend to focus on outward things. We need wisdom in order to keep the sin as sin but to direct the child's attention to the heart. We might say: "Yes, my child, but to direct the child's attention to the heart. We might say: "Yes, my child, but we have sinful hearts, too. We break God's Sabbath command many times when we don't worship God with all our hearts or when we resent having to forgo play in order to attend church. Playing golf, going to see a movie, or mowing the lawn on Sunday are not the only ways to violate the Sabbath.
We should spend time teaching our children the truths of Genesis 1—3, explaining the exalted manner of our creation and the depth of our fall into sin. This will help our children understand that they owe everything to God. In fact, they owe everything to God in a double way, for He has both made them and placed them in covenant with Himself, declaring that He is willing to be their God.
I love the story of the little boy named Henry who complained to his father one day that he was poor and the boy down the street was rich. That boy got an allowance of $5 every week, but poor Henry got only $1. After Henry had expressed his disappointment about this, his father said to him, "Henry, you are a millionaire." Henry said, "What do you mean? I've only got a little bit of money." Pointing to his right arm, his father said, "Son, I'll give you $50,000 for that right arm. Shall we take it off?" The boy said, "Oh, no, Dad; I want my arm." The father said, "What about $100,000 for your leg then?" Henry said, "Oh, no." After going through various other important body parts and adding up their worth, they discovered that Henry was in fact a multimillionaire. Then the father said: "You know, Henry, God gave you all these parts, and you owe them all to God. You must use them all to God's glory; that is why He created you. Every time you look at and meditate on something sinful, every time you go into a wrong place, every time you do something wrong with any part of your body, you are sinning against God." Illustrations like this help children understand that they owe everything to God.
Challenging our children to recognize internal sin is tough because they think in concrete terms instead of abstract ones. So, we must offer them some practical illustrations. For example, we can show them a cup that is perfectly white on the outside but dark and dirty on the inside, then say: "I want you to look at the cup. This cup shows us what we are. We try to make ourselves white on the outside, but inside our hearts are black with sin. We are selfish, we are proud, and we are unbelieving sinners. Just look at that cup and realize that you are trying to look good on the outside but are wicked on the inside."
This excerpt is taken from Parenting By God's Promises by Joel Beeke.