I think one point of encouragement would be that salvation is of the Lord. No parent can guarantee the salvation of their own child. When the book of Proverbs says, “Train up a child in the way he should go, and when he is old he will not depart from it” (Prov. 22:6), that is simply a general observation; it is not a promise—that would be to misinterpret Hebrew wisdom literature.
Parents simply need to know that, before God, we have done the best we knew to do at the time, we looked to God, and we tried to bring Christian influence to bear upon our children’s lives. But we cannot coerce our children into the kingdom. We cannot manipulate them into the kingdom. There is only One who can bring them into the kingdom, and that is God the Holy Spirit. We must have a realistic understanding that salvation is of the Lord, not of man.
Every parent wants their children, longs for their children, and prays for their children to be in Christ. But at the end of the day, no parent can save their child, and no parent can give repentance and faith to their child. God must do that work of grace. So, my encouragement would begin there.
Second, I would say that the story is not over. A child who becomes a prodigal may yet come home, and a gracious Father will receive them, clothe them with the righteousness of His Son, and wash their sins away. Until that child is in the grave, there is still hope that God will yet work.
The third thing I would say is that God will do what is right, and God owes salvation to no one. He says, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion” (Rom. 9:15), and, “Jacob I loved, but Esau I hated” (Rom 9:13). A great encouragement for every parent is that God will do what is right. It should amaze us that any child is saved and converted, and they become a trophy of grace. We can understand the wayward child, but it takes divine intervention for God to rescue a child and bring them to Himself.
So, I would say those three things by way of encouragement. Continue to pray, continue to love, and continue to reach out, knowing that the end of the book has not yet been finished. God may yet bring them to Christ.