I working on it - albeit very slowlyIMHO, Harry Phipps was not involved. He was investigated multiple times, including after allegations by one of his own sons, yet still no evidence was found.
It seems Bevan Spencer Von Einem can also be ruled out unless he was involved as part of a wider gang. He wanted teenage boys and young men, not young kids, if I am not mistaken. That rules out his interest in girls (Jane and Arnna), and Grant Beaumont would be too young for his tastes.
Tony Munro also wanted teenage boys and would have no interest in the Beaumont girls or a male as young as Grant Beaumont.
Whoever was seen at the beach with the Beaumont kids was not old - maybe 20s, or late 30s at most. That rules out a perp like Arthur Stanley Brown.
It's a fascinating case, but I don't think anyone has a clear idea or any really good leads.