child's play vs recreation

child's play and recreation both are nouns.

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

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

child's play: activity by children that is guided more by imagination than by fixed rules
recreation: an activity that diverts or amuses or stimulates

Other hypernyms of child's play include diversion.

child's play (noun)	recreation (noun)
any undertaking that is easy to do	an activity that diverts or amuses or stimulates
activity by children that is guided more by imagination than by fixed rules	activity that refreshes and recreates; activity that renews your health and spirits by enjoyment and relaxation