In English, "child's play" can literally mean "play performed by a child" or it can be used as an idiom to refer to "something easy".
I see that Japanese 児戯 can also mean 子供の遊び (児の戯れ, I guess?) - but can it also be used as an idiom to refer to "something easy"? I suppose you could always use a "simile"-type construction, like 「その試験は児戯のように簡単だった」, but would I be clearly understood if I were to say something more terse like 「その試験は児戯だった」 (cf. slightly unnatural English "That exam was child's play")?