Earlier quoted context omitted.
I think my problem is that it reacted after seeing the child step out from behind the SUV. An excellent driver would have already seen that possible scenario and would have already slowed to 10 MPH or less to begin with. (It's how I taught my daughter's to drive "defensively"—look for "red flags" and be prepared for the worst-case scenario. SUV near a school and I cannot see behind it? Red flag—slow the fuck down.)
I don't see how that's feasible without introducing a lot of friction. Near my house, almost the entire trip from the freeway to my house is via a single lane with parked cars on the side. I would have to drive 10 MPH the entire way (speed limit is 25, so 2.5x as long).
A single lane residential street with zero visibility seems like an obvious time to slow down. And that's what the Waymo did.