Whoever speaks the truth gives honest evidence,but a false witness utters deceit.
(Proverbs 12:17 ESV )
Truthfulness is another virtue which can be imposed upon children, they soon learn that if they lie and get found out the consequences are not nice. But the risk is that some kids will also learn that the way to beat this system is to not get found out. Better by far is for truthfulness to dwell in their hearts.
This is a big ask because humans are deceptive by nature, even our hearts are deceitful (Jeremiah 17:9). The society we live in is deceptive, purposely and powerfully portraying images of ideals that are far from the realities they are masking. Then there is our enemy, who is and always was a liar (John 8:44).
With all these influences arrayed against them, a child will not automatically be truthful. We need to teach our kids what truth is and plead with God that He will lead them in all truth and sanctify them by the truth (John 17:17).
The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
(Luke 6:45 ESV)