Be small, very small. No more than two years old, three at the most. For older children are little rascals who already want to deceive their parents with bare-faced lies.
It is because they have the inclination to sin, fomes peccati but they lack the experience of evil, which will teach them the science of sinning and show them how to lend an appearance of truth to the falseness of their deceits.
They have lost their simplicity, and without simplicity it is impossible to be a child before God.