I've had kids forget a thing or two over the years during daytrips or portages.  Nothing of great consequence.  But I agree with others as to why someone would take something as critical as a portage yoke and shoes...

I recall a couple years ago on a solo, I was just getting ready to push off on the last of a 10-portage day in July with temps mid-80s.  I was bushed.  As I put the last pack into the canoe and looked for my rods/reels, I realized I had left them at the end of the end of the previous portage.  Arrgghhhh!!   >:(  >:( So I had to unload the canoe, carry the canoe back to the pond, paddle across the pond to retrieve my rods and then return to the end of the 10th portage.  Now I was really bushed.  Thankfully, the rods were still there.  Not sure what I would have done if someoner had picked them up....  :-/