Jay Younts at Shepherd Press gives some helpful thoughts on connecting the forgiveness of Christ with the sin of our children AND our own sin. The gospel is not merely about sin management or behaviorism.
We apply the gospel to our children’s sin in the midst of their disobedience. And we identify with them in their sin. We are both an authority to be obeyed and fellow sinners who are tempted just like them.
Read the whole thing. The dialogue can be modified depending on the age of your child.