I like the farm labor argument for the demographic shift. I think the main thing is that the marginal benefit of additional children is huge when you're using them for labor, while the marginal cost of additional children is huge when you're putting them through college. One college-educated child who will almost certainly survive to adulthood is about as useful as five such children, and way cheaper. If they're not getting college education and not surviving to adulthood and performing farm labor from a young age, you need more of them.