1
general::
Something easily done, a trivial matter. For example, Finding the answer was child's play for Robert, or The fight we had was child's play compared to the one I had with my mother! Originating in the early 1300s as child's game, the idiom was already used in its present form by Chaucer in The Merchant's Tale: "It is no child's play to take a wife."
American Heritage Idioms
2
general::
Phrase(s): child’s play
something very easy to do. • The test was child’s play to those who took good notes. • Finding the right street was child’s play with a map.
McGrawhill's American Idioms And Phrasal Verbs