Google “Plural possessives” – you might get a tutorial that will help. Or try this one – http://grammar.ccc.commnet.edu/grammar/possessives.htm

Mary’s–apostrophe after the S would mean more than one Mary.

children’s–Although “children” is plural, it’s one of those funky words in English that there really is no trick for.

children is the plural, there is no such thing as childrens. use the ‘s to signify ownership.

Mary’s grandchildren

same with mary. it’s a name, there’s only one of her. just use apostrophe s (‘s) to indicate ownership.

three girls’ coats

girls is the plural. since it already has an s, you don’t add a ‘s, you just add the apostrophe.

steve and mary’s wedding

it’s a compound noun, “steve and mary”. together, it’s their one wedding. (hopefully)