Because the word the " childrens " does not exist. 'Child' is the singular form, and 'children' is the plural. We add an 's' directly on to the end of a noun to make a plural e.g. boy -> boys To make a genitive/possessive, we add an apostrophe and then an 's' e.g. boy's When a word is already plural, we make the genitive form by just adding an apostrophe at the end: e.g. boys', girls', men's, women's, children's

I see, thank you very much!

Whose holiday? The children's (holiday). 's=belonging to (possessive form) ex: Mom's books (Books that belong to my Mom) "Childrens" does not exist. The world children is already plural so all we have to do is add 's. Hope this helps. Cheers!