post office vs child's game

post office and child's game both are nouns.

Word	Noun	Adjective	Verb	Adverb
post office	Yes	No	No	No
child's game	Yes	No	No	No

As nouns, child's game is a hypernym of post office; that is, child's game is a word with a broader meaning than post office:

post office: a children's game in which kisses are exchanged for pretended letters
child's game: a game enjoyed by children

post office (noun)	child's game (noun)
a children's game in which kisses are exchanged for pretended letters	a game enjoyed by children
a local branch where postal services are available