This is a reason I have the habit of walking around my vehicles before I get in them. I live across from a elementary school so I do all I can to make sure that no child is behind any of my vehicles before backing. I back into my drive a lot too. This helps because I do not have to worry as much about what is behind my vehicle. I still walk around it before getting in though.
Oh my goodness, that must be so heart breaking for the parents they will live for years with the guilt of it. Some cars have cameras on the sides and back, but really before you leave home you should make suree all your kids are in the house.