child's play vs cinch

child's play and cinch both are nouns.

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

As nouns, child's play and cinch are synonyms defined as:

Other synonyms of child's play include breeze, duck soup, picnic, piece of cake, pushover, snap, walkover.

child's play (noun)	cinch (noun)
any undertaking that is easy to do	a form of all fours in which the players bid for the privilege of naming trumps
activity by children that is guided more by imagination than by fixed rules	any undertaking that is easy to do
	stable gear consisting of a band around a horse's belly that holds the saddle in place