2. A knowledge of the father's part
When it becomes necessary, you must also tell them of the father's part; that is, of the marriage act.

This is the advice the Pope gives you: "...you will not fail to watch for and to discern the moment in which certain unspoken questions have occurred to their minds and are troubling their senses. It will then be your duty (mothers) to your daughters, the father's duty to your sons, carefully and delicately to unveil the truth as far as it appears necessary, to give a prudent, true, and Christian answer to those questions, and set their minds at rest. If imparted by the lips of Christian parents at the proper time, in the proper measure, and with the proper precautions, the revelation of the mysterious and marvellous laws of life will be received by them with reverence and gratitude, and will enlighten their minds with far less danger than if they learned them haphazard, from some disturbing encounter, from secret conversations, through information received from over-sophisticated companions, or from clandestine reading, the more dangerous and pernicious as secrecy inflames the imagination and troubles the senses. Your words, if they are wise and discreet, will prove a safeguard and a warning in the midst of the temptations and corruption which surrounds them." ⁹

When to give it
It is for you to watch your children carefully. Any time from about ten onwards they may need this extra information. "The sex-instinct awakes in different children at different ages" (the Scottish Bishops). Even if they are too shy to ask, you should be ready to speak and answer their "unspoken questions".

Now supposing your children show no signs of trouble at all.

⁹ The Pope Speaks to Mothers.