WOW!!! I must be the world's worst Mom!!!
My 3 year old plays in the back yard by himself, and sometimes in the fornt yard /driveway when it rains and he is puddle splashing!!
I do check him from the window every couple of minutes, or in the summer when the windows are open, I make sure I can hear him.
We do live on a dead-end street with only 15 houses.