I really find it dubious that graphic is down to cars at all, really. Like even if the swimming pool backed onto my house I wouldn’t (couldn’t) let my 8 year old daughter go there on her own.
Really it’s just another example of weird old people convinced that there’s too much health & safety and kids aren’t allowed to be kids any more. Mostly it’s that people probably have the luxury of not leaving their kids alone. Certainly my single mum didn’t leave me alone during the holidays when she went to work or to walk to primary school and back because she thought it was good for me to have independence, it was because she had no other option!
(I’d also guess the population differences of 100 years ago and the population demographics post WW2 mean comparisons to 1919 and 1950 are fairly dubious.)
Regarding the image I posted, I’m not saying cars aren’t dangerous but when you choose to illustrate the danger of cars by implying that to put a foot on the road or off a crossing will result in instant death you’re clearly being hyperbolic and your satire fails to really have any strong meaning.