There is nothing like the joy which comes from children who have been faithful. John was speaking figuratively of the saints but the principle is the same when he said, I have no greater joy than to hear that my children walk in truth (3 Jn 1:4). Lo, children are an heritage of the Lord: and the fruit of the womb is his reward. As arrows are in the hand of a mighty man; so are children of the youth. Happy is the man that hath his quiver full of them (Ps 127:3-5).
On behalf of parents who have worked so hard for the salvation of their children, let's rewrite DC 18:15-6 as follows: And if it so be that you should labor all your days in [raising your children], and bring, save it be one soul unto me, how great shall be your joy with him in the kingdom of my Father? And now, if your joy will be great with one [child] that you have brought unto me into the kingdom of my Father, how great will be your joy if you should bring many [children] unto me!