Which kind of noun is this?

children's

Referring to one person = singular

Referring to two or more = plural

What does the 's at the end tell you?

It is plural

http://www.english-for-students.com/declension-of-nouns.html

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Plural what?

The noun "children's" is a possessive noun. It indicates that something belongs to or is associated with children. In this case, the possessive form is used to show that the noun following the apostrophe belongs to or is related to children. To identify the type of noun, you can look for possessive markers like apostrophes ('s) or the word "of."