True, but I'm the context of the film, it was set in (then) current day, and at least partially tried to make him being left behind plausible in that context.

Though honestly, with the way they showed the mix-up even back then, it's plausible the same thing could happen with a kid now... If they look similar enough, and the parents were seriously distracted, it's not like they have id for the kids or anything now. It's parents dragging kids along and once you're past security it's basically the ticket agents glancing to make sure the number of people and number of tickets match.