I wonder if that is a "fully attentive human drive who drove exactly the same as the Waymo up until the point the child appeared"?

Personally, I slow down and get extra cautious when I know I am near a place where lots of kids are and sight lines are poor. Even if the area is signed for 20 I might only be doing 14 to begin with, and also driving more towards the center of the road if possible with traffic.

I do the same, and try to actively anticipate and avoid situations like this. Sadly, in my experience most drivers instead fixate on getting to their destination as fast as possible.