possessive case with plural

How can I use the possessive case with many objects " s’ "?
2Jane and 2Mary 2room;

it is right? :

girls’ books

Jane and Maries’ rooms

the children’s toys

the workmen’s tools

Peter and Helens’ flats are large

10Jane and 10Mary 10*room;

Jane and Maries’ rooms
or/and
Janes’ and Maries’ rooms