What is the rule of plural possessive?

Answer 1

If the word is already plural, such as children, add an apostrophe and then the s, such as "the Children's Books". This is also the case with plurals which are formed when an s is added, e.g. the dogs' kennels. If the word is not already a plural, the apostrophe goes before the s, e.g. the dog's kennel.

Answer 2

The rule for plural possessive is to add an apostrophe after the ‘s’ when a plural noun ends in ‘s’, such as “The students’ books.” If a plural noun does not end in ‘s’, add an apostrophe followed by ‘s’, like “The children’s toys.”