child's play vs task

child's play and task both are nouns.

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

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

Other hypernyms of child's play include labor, labour, project, undertaking.

child's play (noun)	task (noun)
any undertaking that is easy to do	a specific piece of work required to be done as a duty or for a specific fee
activity by children that is guided more by imagination than by fixed rules	any piece of work that is undertaken or attempted

child's play (verb)	task (verb)
	use to the limit
	assign a task to