The top reply the the top comment shows exactly why this "the number 1 preventable cause of death" nonsense doesn't actually line up with reality. Traffic deaths decrease while population is increasing, street are demonstrably safer for children than they ever have been. Barely 100 deaths for 330+ million people in the US, likely even less in less car-centric cultures.
That's true but the number is exceedingly small. 4000 deaths / a couple hundred million is pretty much as good as it's going to get. That's just what happens with really big numbers.
Edit: and that's car accidents, not pedestrian accidents which is also a fraction of that 4k.
Ok multiply it by 10 and it's still ~1000 deaths. 100 / 100 million kids or whatever it is is pretty goddamn safe. You'd need to turn every road into a tunnel with gates to bring that number down.
Reducing the speed limit further is not going to help because there is barely enforcement of the speed limit as it is today. You could reduce it to 10mph everywhere, and people will still drive the same speed as they do today.
I know you are saying that in jest, but that would clearly not make kids safer. :D Just think of the number of kids who would be shot because someone was trying to stop a speeding car. (either kids in the car, or behind the car, or around cars)
Step 1 is to reduce speed limit to 45kph (25mph) anywhere a pedestrian could interact with the road which is what Vision Zero recommends.