Disappearance of the Beaumont Children

In 1966, three Beaumont siblings disappeared from a beach in Australia. There were kidnapped at the same time, in a busy public place, with very little conclusive evidence about what had happened. As the case started picking up heat in the media, there were a lot of weird complications (possible sightings, hoax ransom notes/calls, etc.) and potential connections to other kidnappings and murders. They were never found. Interestingly, it's widely considered to be a big turning point for attitudes toward children in Australia. After the Beaumont children disappeared, people were much less willing to let their children out unsupervised, though, of course, it wasn't the first time a child had been kidnapped by a (presumed) stranger.