I hate I know this and I know it's a joke but here we go
In theory, they all become -
Mrs. Afton - Ballora
Elizabeth - Circus Baby
William Afton - Doesn't really become anyone but he is inside Springtrap
The Crying Child - Theorised to be Golden Freddy
Michael Afton - Doesn't become anyone at all, but does have robot spaghetti of all the Sister Location animatronics// his friend is Eggs Benedict but no one knows for sure.