We are on the same page WRT the public school system, but my point remains. The money spent on today's children are not an "investment" by the working population of today. To make that point valid we would have to be expected to not compensate for the services/products that those children will provide when they are of age.
Anyone getting an education is making an investment in themselves and hoping to increase their future value. I will compensate a doctor more for their services than I will compensate a bartender. That is true regardless of my age. If the family does not teach their child that education is important, then their potential value is cheapened.
The "society invests in children" side of this argument is only denying that they are responsible for ensuring that their children know how to maximize their future value. This argument actually is worse for our future than the individualists expecting to fairly compensate a service provider in the future.
I should like to aid young people in learning, even if I am not their paid teacher and even if they are not "my" young people. Likewise they should expect to learn from elders, and feel themselves bound in some way to honor elders, just on the basis of a general, intergenerational respect.
The ties that bind us to each other, person to person, gender to gender and generation to generation, are mostly customary rather than legal. We owe so much to others just by being begotten and born, that we could never think to put most of our relations on a pay-for-service or "enforceable" basis.