IMO , there are two thoughts regarding Foster Father coming out of the Bathroom seem to be crying.
I repect people theories yes it is strange and may did he know what happened to William.
IMO
September is Spring, people can get Hayfever even I when I visit other places, my medication is tablets and cream but my Understanding Partner takes Nasel Spray, he coughs, sputters even gages, comes out with red crying eyes. Even when gardening he eyes get red and watery due to Pollen. I get breathy/huskey and people think I doing a bad Munroe performance .
The other is taking a leaf from my past with my first foster family(Who I am still very close altough lived distance away) I went missing briefly as a eight year old. I wandered off on the Farm, misplaced my steps as I was swooped by a Magpie,(Nesting Time) tried to get back before sunset I did not I paniced could not moved was found sitting in the dark,years later foster family told me how my Foster Father who is to this day is a quiet calm spoken stoic man who was shaken and so was my foster Mother even within an hour I went missing he had concerns I could been bitten by a snake, fallen down some where etc. I had never wander off before always stayed close to the farm house. He said years later he dreaded having to make the Phone call to my Welfare Officer to report I was missing although I was found few hour later, he and my foster mother still were questioned by Child Services about what happened.
Even if William had been found, (in my heart I wished it happened) the Foster Family would been interveiwed about the incident. There would been possible the children taken away briefly or more.