> The only thing that still has me puzzled is when to put an apostrophe after the word ie childrens', girls', Charles' (or Charles's), clothes.
It's simple enough, SG.
Children is already plural - it's the plural form of 'child' - so the children's clothes were on the floor.
Girl's = belonging to the girl
Girls' = belonging to the girls
Charles ends with an 's' so Charles' clothes were on the floor. Cf you can only get to heaven in Jesus' name, etc.
As for clothes, again that's easy because there's no singular form of this noun i.e. clothe. Therefore, the clothes' colour had faded over time.