>In the US you can mow down a child, drive away, and despite people having your plates and giving them to the cops, they can't actually arrest you because it was only your car which was used to kill someone?

Not quite. In the US you get in trouble for driving off, but drivers that wait for the police to show up and then blame the child that they mowed down have a decent shot at having zero consequences, especially if the child was riding a bicycle.

https://nextcity.org/features/how-much-is-a-cyclists-life-wo...

https://www.latimes.com/opinion/livable-city/la-oe-schultz-p...

https://www.nytimes.com/2014/09/30/opinion/why-drivers-get-a...