Train them to have a habit of always speaking the truth.
Speaking the truth is far less common in the world than we may suspect. The whole truth, and nothing but the truth, is a golden rule which many would do well to bear in mind. Lying and deception are old sins of the past. The devil was the father of them—he deceived Eve by a bold lie, and ever since the fall of man it is a sin which all the children of Eve have to guard against.
Think about how much falsehood and deceit there is in the world! How much exaggeration! How many untruths are added to a simple story! How many things are left out, if it does not serve the speaker's interest to tell them! How few there are around us of whom we can say, that we trust their word without question! Truly, the ancient Persians were wise in their generation: it was a most important rule with them in educating their children, that they should learn to speak the truth. What an awful evidence of man's natural sinfulness, that we should have to mention such a point at all!
My friend, think how often God is spoken of in the Old Testament as the God of truth. Truth seems to be especially set before us as a leading feature in the character of Him to whom we are subject to. He never swerves from the straight line. He abhors lying and hypocrisy. Try to keep this continually before your children's minds. Press upon them at all times, that anything less than the truth is a lie; that evasion, making excuses, and exaggeration are all halfway houses towards what is false, and ought to be avoided. Encourage them in any circumstances to be straightforward, and, whatever it may cost them, to always speak the truth.
I bring this subject to your attention, not merely for the sake of your children's character in the world, but rather for your own comfort and assistance in all your dealings with them. You will find it a great help, to always be able to trust their word. It will go far to prevent that habit of concealment, which so often prevails among children. Openness and straightforwardness depends a lot on a parent's treatment of this matter in the early days of our childhood.
The book "The Duties of Parents" can be found here: J.C.