Train them to have a habit of faith.
By this I mean, you should train them to believe what you say. You should try to make them feel confidence in your judgment, and respect your opinions, as better than their own.
You should cause them to think that, when you say a thing is bad for them, it must be bad, and when you say it is good for them, it must be good; that your knowledge, in short, is better than their own, and that they may rely implicitly on your word. Teach them to feel that what they do not know now, they will probably know later, and to be satisfied there is a reason for everything you require them to do.
Who can describe the blessedness of a real spirit of faith? Or rather, who can tell the misery that unbelief has brought on the world? Unbelief made Eve eat the forbidden fruit—she doubted the truth of God's word: "You will surely die." Unbelief made the old world reject Noah's warning, and so perish in their sin. Unbelief kept Israel in the wilderness—it was the barricade that kept them from entering the promised land. Unbelief made the Jews crucify the Lord of glory—they did not believe the voice of Moses and the prophets, even though they were read to them every day. And unbelief is the reigning sin of man's heart down to this very hour—unbelief in God's promises—unbelief in God's wrath and discipline—unbelief in our own sinfulness—unbelief in our own danger—unbelief in everything that runs counter to the pride and worldliness of our evil hearts. Your training of your children is worth very little if you do not train them to have a habit of implicit faith—faith in their parents' word, confidence that what their parents say must be right.
I have heard it said by some, that you should require nothing of children which they cannot understand, and that you should explain and give a reason for everything you desire them to do. I solemnly warn you against such a notion. I tell you plainly, I think it is an unsound and corrupt principle. No doubt it is absurd to make a mystery of everything you do, and there are many things which it is good to explain to children, in order that they may see that what we say is reasonable and wise. But to bring them up with the idea that they must take nothing on trust, that they, with their weak and imperfect comprehension, must have the " why " and the "wherefore" made clear to them at every step they take—this is indeed a fearful mistake, and likely to have the worst effect on their minds.
At certain times, if you are so inclined, reason with your child, but never forget to keep in mind (if you really love him) that he is only a child—that he thinks as a child, he understands as a child, and therefore must not always expect to know the reason for everything.
Set before him the example of Isaac, in the day when Abraham took him to offer him as a sacrifice on Mount Moriah [Genesis 22]. Isaac asked his father a simple question, "Where is the lamb for the burnt offering?" and he got no answer but this, "Abraham answered, "God himself will provide the lamb." How, or where, or when, or in what manner, or by what means—all this Isaac was not told; but the answer was enough. He believed that it would all be okay, because his father said so, and he was content.
Also, tell your children, that we must all be learners in all of our beginnings, that there is an alphabet to be mastered in every kind of knowledge—that the best horse in the world once needed to be broken—that a day will come when they will see the wisdom of all your training. But in the meantime if you say a thing is right, it must be enough for them—they must believe you, and be content.
Parents, if any point in training is important, it is this. I charge you by the love that you have for your children, use every means to train them to have a habit of faith.
The book "The Duties of Parents" can be found here: J.C. Ryle