Trickier apostrophes

Children will have learnt in Year 2 that 's is added to singular nouns to show possession (the cat's tail).  This is usually the same for a proper noun that already ends in s.  Examples are Charles's car, James's book.  When possession is shown in this way, the additional s is pronounced.  

As with everything in our language, there are exceptions to be found: for example, St. James's Park in London uses the 's, whereas St. James' Park, home of Exeter City FC, just uses the apostrophe on its own.  Very often the exceptions relate to names of places or organisations, so it is a good idea to check the source through an official document or website.

Sometimes names end in s, but to pronounce an additional s would be awkward, e.g. Achilles' heel, not Achilles's heel.  You wouldn't say 'I watched Kevin Bridges's show.' or 'I love Saint Saens's music.' Therefore, in these instances we would just use Kevin Bridges' show and Saint Saens' music.

There is much debate over whether proper nouns ending in a sibilant sound (/s/ or /z/) should take 's or just the apostrophe to indicate possession.  Writers make a choice about which convention they use and children will find exceptions and will delight in pointing these out to their teachers and parents.  I believe, when working with children, best practice would be consistency in adding 's, unless this makes the word awkward to say, or we just wouldn't even consider pronouncing the additional s.