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