If a preschooler hits another child with a rock....,

If all children fear the wrath of the rocks held by their peers, they will think twice before using their own.

Unless of course they are an idiot - in which case they would be swiftly stoned to death by everybody in the vicinity.

Win-win situation..

I disagree, the threat of reciprocal violence does not prevent violent acts. One has only to observe prison populations to illustrate this point. Next?

It doesn't prevent all violent acts - but it does thin the violent out of the population. Scraping the scum off the gene pool.

Prison? Prisons use resources which would be of better use to hospitals, schools, libraries and so forth. No need for them.

Ah but if you give a preschooler a shiny vest and a sling shot to launch the rocks and tell everyone they will be shot if they don't listen to the kids with slingshots then most of them behave but a few break the rules and throw their rocks anyway. Eventually the kids with slingshots will realize they can shoot the other kids anyway, then they make the other kids give up their rocks and the only ones left with rocks are the kids with slingshots and the kids that break the rules and pick up rocks off the ground to throw.