How do they know?

Psalm 71:17 "Since my youth, O God, you have taught me, and to this day I declare your marvelous deeds.”

This scripture implys that someone during this person's youth was teaching them about God. And guess who? Correct answer: parents. There are many ways in which we can share the "marvels" of God to our children. The most obvious one is the ENTIRE physical world. Remember kids are constantly learning and forming opinions about the world they live in, make sure God gets the credit for the things "he hath done."