If the mother left the kids unsupervised for just a few minutes, as she claims.........then common sense tells you that they couldn't have gotten very far. Therefore, why didn't someone OPEN THE DAMN TRUNK. After all, it wasn't a stranger's car parked in the street. It was the grandmother's car parked on the property (side or back yard), where the kids were last seen playing.
Or the mother is lying and left them unsupervised for a much longer amount of time. That could lead her to think they wandered off.
Either way, mom's at fault. IMO
You have said everything I was trying to say, and better.