If the parent had said 15 or 20 mph instead of 25, there wouldn't be a significant difference in traffic deaths. If you get hit by a car at 15mph, you are highly unlikely to die. You may break a bone, but you won't be dead.
Of course, that excludes freak accidents -- you could fall onto a spike or something. But freak accidents are possible whether the car is going 5mph or 15.
