doddle vs child's play

doddle and child's play both are nouns.

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

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

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

doddle (noun)	child's play (noun)
an easy task	any undertaking that is easy to do
	activity by children that is guided more by imagination than by fixed rules