If you cannot see kids by looking at your rear window and using your side mirrors (don't know how they're called in english), your clearance might be too high and your mirrors poorly set.
When i'm driving my father's utility vehicle, i can only use my side mirrors, so i'm extra careful, but with any normal car i ever drove, even large one like a Xantia, i don't see how i could have missed anybody who can move on two legs.
The only time I've used a rearview camera was on my SO "new beetle", and we both find it quite useless (i was seeing better turning my head).
https://www.consumerreports.org/cro/2012/03/the-danger-of-bl...
I have never seen any car where it is possible. The entire rear section of the vehicle would have to be transparent and any passengers or cargo behind the driver would have to turn invisible when you went into reverse.