If you need some basic information about the possessive apostrophe, go to: The possessive apostrophe. Part 1 – the basics.
If you already know that stuff, you’re ready to look at where the possessive apostrophe is placed when dealing with words like ‘children’.
To be technical, ‘children’ is an irregular plural noun. In other words, you don’t stick an ‘s’ on the end of ‘child’ when you want to talk about more than one child. Instead, you use a completely different word: ‘children’.
Other irregular plural nouns include:
Women
Men
Mice
oxen
In all such cases, the possessive apostrophe is followed by an ‘s’, for example:
The children’s meal was late.
You will see this written as “childrens” (no apostrophe) and “childrens'” (with an apostrophe after the ‘s’). Both of these are wrong.
Similarly, the following are correct:
The women’s complaints were ignored.
The men’s work was very hard.
The mice’s home was blocked up.
The oxen’s owner had disappeared.
Wordwatch Towers 