Something easy to do: writing a novel isn't child's play(1380s+)
Idioms and Phrases with child's play
child's play
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.”