Why do you assume kids are problems to begin with? I have met hundreds of kids during my adult life and the highest any of them has risen to for me is minor annoyance. I've had more trouble from my cars.
I'm not the one that used "problem" to describe kids, btw. FWIW I quite like kids. But saying strangers kids are my responsibility because of "society" is not true at all.
Kids are a part of society, and society has a stake in what kind of adult they become.
In the same way drug addicts are society's problem, or unemployed people are society's problem, or sociopathic second generation rich never-worked-a-day drunk drivers are society's problem.
This isn't to say that you'll be conscripted in to a psudo-parent role against your will, but they are your responsibility as a member of society in a very abstract way.
We detached this subthread from https://news.ycombinator.com/item?id=14128052 and marked it off-topic.