Re: Grammar please
There is nothing ungrammatical about the original. "their" is a third person singular possessive pronoun.
On this subject, I was reading the biography of Helen Keller the other day (as you do) which was written roughly a century ago and I was struck by the consistent use of "him" and "his" to refer to a child when the passages, though couched in general terms, were clearly talking about Helen's childhood and Helen's teacher's particular approach to educating her. Certainly in my part of the world, such usage is now almost laughable.