It’s likely that an unsupervised child will find something dangerous to do, especially while in a public place. I worked at a school that had PE in a public park adjacent to the school, and we always had to watch all of them like a hawk. Some teachers just sit on the side and chat instead of really focusing on the kids, and I’ve always been irked by that. There was an incident where a child took home a hypodermic needle that he found, which the nanny discovered while doing the laundry. Watching 20+ kids at a time is difficult, but you have to do it ALL THE TIME. Even watching 4 kids is difficult, but you have to do it ALL THE TIME. There are many small things in life that are easy to mess up if you’re careless for just a second. Driving and child care are especially fraught with danger. Not paying attention in a public space is not acceptable.