BBM.
That's exactly the impression that I have. I don't think he took William, but I suspect he saw what happened.
For an elderly couple living in a quiet cul-de-sac in a country town, to have kids from Sydney visiting the neighbour (FGM) would have been a huge deal.
William hadn't been there long before he disappeared. It was his first morning at FGM's house. The kids were running around yelling in the morning. It would've been totally out of the ordinary for PS to hear those new sounds in the morning. He would've been watching and listening to them for sure.