Note that the movies this convention is based on are themselves based on the very real issue of 'latchkey kids' that was prevalent (at least in the USA) through the 1970s and 1980s. The stereotypical situation was that the Boomer parents were more interested in whatever they were doing and let the kids run around doing whatever. So there are a lot of movies in the early 80s where kids who are out on their own go on an adventure.
You can even go back to the 1950’s and “The Blob” for a story of kids (well, older teens rather than tweens in that case) having to fight off an otherworldly menace because the adults are all too oblivious or clueless to realize what’s going on.
It may be on a slight upswing from its lowest point, but I think violent crime in general is still down compared to the kids-on-bikes decades.
Definitely true that (in the U.S.) “kill a bunch of people in a public place” has become a much more common way to express anger at the world than it was in the past, though as noted above it is statistically still rare compared to far more mundane ways to get hurt or killed.