Changes, doesn't it. My 1960s' childhood my parents were intentionally hands off - we'd roam for miles, ride bikes on the Great north Rd when we were five, climb trees when our parents were well out of sight, jump off 15 foot walls... But by the time I was a parent, things were different. (I had a hole in the heart and my parents decided to let me be active, rather than end up in a wheelchair as many kids with my defect did, in those days). Like most kids of my generation, we played out til it was dark. For us "playing out" might mean being 2 miles away in the woods, or right the other end of the village, or playing by a pond (when I couldn't swim) or anything. We were essentially feral!
ETA: That said, my mum would never have let me out of her sight when I was a baby/toddler of those children's ages.
What always puzzled me about this particular thing was that they were GPs so presumably when junior housemen or students, had worked in A & E. They'd know better than anyone the dangers unattended kids could get into. Added to that, one of the other kids of the tapas bar party, had had/was having a stomach bug - and anyone with young kids knows that those things rip through the entire group if you have a herd of kids. Who'd leave a kid who might start throwing up in the night, alone? Let alone a dr?
Someone in a neighbouring flat allegedly said they'd heard Madeleine crying when left alone another (the previous?) night, for an hour, and almost rang the police. So the child wasn't happy about being left alone there. Kids sometimes don't sleep well in strange places so even if she was asleep before they left, it wasn't a kind, or the sensible, thing to do. Because they'll have known, from the previous night, that she might wake up to find herself alone, and be afraid. (I feel sorry for the woman who has had to live with the guilt that she didn't call the police, that night, too as that might have averted what happened if the coppers had shown up the night before?)