My brother would have let my parents drive to the other side of the world without mentioning me not being in the car if this had been us when we were little. He would have been delighted that the holiday was already going better than he could have hoped 
My mum did actually leave me in my pram outside a shop once when I was a baby, but we lived less than a minute away and she remembered me as soon as she got home. She ran straight back.
I imagine that both parents thought the other had strapped the child in and then perhaps thought she was asleep.
I read that they only realised she was missing when they heard on the car radio that a child had been found at the service station they had stopped at.
My heart goes out to them for the total panic they must have felt at that point.
I can't imagine being in that situation, having just DS think of, but I hope i'm never distracted enough to find out.