You must not expect to find your children's minds a sheet of pure white paper, and to have no trouble if you only use right means. I warn you plainly you will find no such thing. It is painful to see how much corruption and evil there is in a young child's heart, and how soon it begins to bear fruit. Violent tempers, self-will, pride, envy, irritability, passion, idleness, selfishness, deceit, cunning, lying, hypocrisy, a terrible aptitude to learn what is bad, a painful slowness to learn what is good, a readiness to pretend anything in order to gain their own ends—all these things, or some of them, you must be prepared to see, even in your own flesh and blood. In little ways they will creep out at a very early age; it is almost startling to observe how naturally they seem to spring up. Children require no schooling to learn how to sin.
But you must not be discouraged and depressed by what you see. You must not think it a strange and unusual thing, that little hearts can be so full of sin. It is the only inheritance which our father Adam left us; it is that fallen nature with which we come into the world; it is that inheritance which belongs to us all. May the awareness of it make you more diligent in using every possible means which seem most likely, by God's blessing, to counteract the evil. Let it make you more and more careful, so far it lies with you, to keep your children out of the way of temptation.
Never listen to those who tell you your children are good, and well brought up, and can be trusted. Rather, remember that their hearts are always ready to burst into flame like dry tinder. At their very best, they only need a spark to ignite their evil. Parents are seldom too cautious. Remember the natural depravity of your children, and be careful.
The book "The Duties of Parents" can be found here: J.C. Ryle