fireman vs child's play

fireman and child's play both are nouns.

Word	Noun	Adjective	Verb	Adverb
fireman	Yes	No	No	No
child's play	Yes	No	No	No

As nouns, child's play is a hypernym of fireman; that is, child's play is a word with a broader meaning than fireman:

fireman: play in which children pretend to put out a fire
child's play: activity by children that is guided more by imagination than by fixed rules

Other hypernyms of fireman include play.

fireman (noun)	child's play (noun)
play in which children pretend to put out a fire	any undertaking that is easy to do
a member of a fire department who tries to extinguish fires	activity by children that is guided more by imagination than by fixed rules
a laborer who tends fires (as on a coal-fired train or steamship)
a pitcher who does not start the game