The point is not that [bad thing] isn't bad. It's that scolding is the wrong way to go about it. How would your friend's dad have reacted to a stranger at the gas station yelling at him about the kids in the bed?

A better approach is friendliness, and gently pointing out how dangerous it is. "Hey man - careful with those kids. My neighbor died that way growing up - might be worth pulling them into the cab" is more likely to get a "Really? Dang, yea, you're right"