I believe that the reason you put an apostophe at the end is that strictly speaking it is indicating that another s follow but that is optionally dropped.
So dogs' bones and dogs's bones are equivilent and strictly speaking both acceptable usage.
However more recently we have started to drop the latter case and that is dying out
so childrens' would be short for childrens's which would be wrong
I had the most awful primary school teacher who was a demon about apostrophes.
As it was absolutely dinned into me , I am totally confident that, in the words of that dreaded teacher (imagine the loudest shouting possible)
Men, women and children are all apostrophe s
men's
women's
children's
And I had to put up with her for a whole year too. It was misery for us all.