From what I can tell, the change comes with motor vehicles, when people have to start monitoring the kids' every moment. Before motor vehicles arrive, you just set them out to play and other adults/older children take care of them.

After motor vehicles, there's a lot more risk to just letting them roam, and now you have to arrange an area where they can play and since there isn't a lot of people milling about outside that can watch them, the parents and other immediate family members shoulder more of the load.