I'm searching around and I can not find those numbers at all.
My pastor has asked the congregation multiple times "how many people have been saved through Sunday School"? It's easily 75%. Sunday School and youth ministry is an important part reaching kids on their level.
You mentioned that it should be the parents. That's a different topic all together. Parents can still be involved in their kids faith development even with Sunday School. That's not a Sunday School issue. that's a parenting issue. Which also might lead to the Sunday School numbers you posted so high. If the parents don't show interest in their kids Sunday School learning, why should the kids care?
I don’t doubt those numbers, but many, many people come back to the church in their early thirties after they are married and perhaps have children.
And they stay. It may be just a season of life issue.