child's play vs picnic

child's play and picnic both are nouns.

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

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

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

child's play (noun)	picnic (noun)
any undertaking that is easy to do	any undertaking that is easy to do
activity by children that is guided more by imagination than by fixed rules	any informal meal eaten outside or on an excursion
	a day devoted to an outdoor social gathering

child's play (verb)	picnic (verb)
	eat alfresco, in the open air