I think you nailed it. Out of site. Sleeping kid. Slips ones mind. Car seats should be allowed on front seat in plain view and this would not happen nearly as often. I know, what about crash protection etc.? The parent should decide front or back.
Tragic, nothing really to say.
As for placement of car seats,
I gotta agree with Ron and Moose on this one Clara.
When you have multiple kids, then one of them has to go in the back. It's not a big deal to have a kid in the back. You can place mirrors in the car to see the kids.
A few years back, I had 3 kids under 3, two of which were babies.