Our children need new hearts (Ps. 51:6–7, 10; John 3:3–5), so we should teach them that we are not dealing with their misbehavior as if they were merely being naughty. We must show them that just being outwardly obedient is not sufficient with God. Good behavior impresses parents and teachers, but God looks on the heart (1 Sam. 16:7).
When a child does something wrong, some parents say, "Well, boys will be boys." That is the worst thing we could say to a boy who has sinned, for it tells him it is all right for him to sin. Rather, when our children sin, we must use that opportunity to teach them about the sinfulness of sin (1 John 1:8, 10) and their need to be renovated from within and receive new hearts (Ps. 51:6).
This excerpt is taken from Parenting By God's Promises by Joel Beeke.