In my opinion, the whole theory about the children being abducted from Glenelg is pure speculation. This theory appears to be based on reports from various people who had seen them with "the man". There has never been any proof at all of an abductioin having occurred.
I think it's important to bear in mind how many people would have been at the beach/Colley Reserve/anywhere else in Glenelg on that day. The weather was absolutely scorching - 104 degrees Fahrenheit/40 degrees Celsius. There would have been literally thousands of people there (in what is a quite small area). Kids would have been everywhere, running around with excitement. Whether the various "sightings" were definitely of the Beaumont children is questionable at best.
As far as I'm aware, the only person who definitely knew the three children, and sighted them on that day, was their postman. He stated that he saw the three children unaccompanied and quite close to their Somerton Park home. Initially he'd thought this was in the afternoon of the day in question, but later, on reflection, thought it could have been morning.
A Wenzel's bakery employee stated that she recognised Jane as being a previous customer when she served her. (For some context here, I lived in Glenelg for several years, and was a frequent customer of Wenzel's Bakery throughout that time. Wenzel's, a small (but iconic) shop, located one street back from the foreshore, was always busy, even on a slow winter's day.
I'm trying to imagine how busy Wenzel's would have been on that day. It was the middle of summer - the height of the tourist season, with the shop absolutely packed with customers. Thousands of people would have been swarming around the Jetty Road precinct, very keen to buy a cool drink, and grab a pie or pasty. Whether or not this employee definitely recognised Jane is by no means definite in my opinion. She would have been incredibly busy serving people, and calculating in her head how much change needed to be given, and then counting it out to customers. (No flash cash-registers in those days.) She would have barely had a moment to even look at customers. The customer who handed over the pound note may not have even been Jane. (Even if it were Jane, it's possible that she found the pound note. It would have been easy for anyone to drop some money by accident whilst trying to navigate the crowds.)
As far as I'm aware, there has been no proof whatsoever of the "abduction" of the Beaumont children. I think it is just as likely that there was an accident by misadventure, causing grave injury or death to one, with the other two dashing to assist, and also finding themselves in trouble. That could be a swimming accident, but I think it's more likely to be something to do with playing in or near drains which emptied into the Patawalonga Boat Haven. (Literally a stone's throw from Colley Reserve.) The drains would probably have been dry, but a sand collapse seems possible. Just my opinion.