Mosiah 26
I had never considered that by the time the various covenant groups of Nephites were united that they had children and youth who were just along for the ride, having not made the covenant for themselves (this was probably mostly true of the people in the land of Zerahemla). The people in the land of Zerahemla had made their covenant about 10 years earlier and some of their young children were approaching adulthood by this time. The people of Alma had made their covenant much earlier, but they had also faced hardship and trial to keep their faith strong so many of their children might well have been strong in the faith. Alma the younger however was an example showing that some of them did not turn to the Lord as a result of their trials. The people of Limhi had only recently been freed from bondage and taken on baptism so I suspect that most of their children were personally converted as well. Alma the younger and his father were a stark representation of this generational split.
The reason that Alma had not had such wickedness among his people before was that they were united in building a new land or being oppressed by the Lamanites for all of their previous history in the land of Helam.