duck soup vs child's play

duck soup and child's play both are nouns.

Word	Noun	Adjective	Verb	Adverb
duck soup	Yes	No	No	No
child's play	Yes	No	No	No

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

Other synonyms of duck soup include breeze, cinch, picnic, piece of cake, pushover, snap, walkover.

duck soup (noun)	child's play (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