Note where the apostrophes occur, and where they don't. The rows show the subject form, the object form, the possessive adjective form, and the possessive pronoun form, respectively.
I you he she we they it Kate the child the boys
me you him her us them it Kate the child the boys
my your his her our their its Kate's the child's the boys'
mine yours his hers ours theirs its Kate's the child's the boys'
Note that there is no her's.
CJ