A quick thought for parents from John Piper. Great reminder to me because I have kids in both camps discussed below.
Jesus tells of two sons:*Italicized words added by me.
What do you think? A man had two sons. And he went to the first and said, "Son, go and work in the vineyard today." And he answered, "I will not," but afterward he changed his mind and went. And he went to the other son and said the same. And he answered, "I go, sir," but did not go. (Matthew 21:28-30)
I draw this advice for parents from these words: Do not despair of the rebel (child #1*), and do not assume the heart of the compliant child (child #2) is with you.
Which means practically, pray like crazy for both of them. And keep showing both of them the gospel of undeserved, blood-bought grace.