child's play vs duck soup

child's play and duck soup both are nouns.

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

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

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

child's play (noun)	duck soup (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