what happened is awful, but I haven't been there at the time and therefore dont know if someone saw the little child. Maybe they were home while he walked past. It's my understanding Australia is populated very thinly, with house spaced very far apart, and it's summertime now and real hot, so people tend to stay indoors with the air conditioner on, drowning every sound.
And when you drive, you dont look at the sidewalk. If the toddler crossed the roads and stayed alive, then perhaps no car went by during those minutes.
You have to be in the same place in the same minute he walks by in order to see him. Five minutes before or after, and you've missed him.