can it also be used as an idiom to refer to "something easy"?-
Yes. As the link @oldergod posted on comment suggests, it also has nuances like puerility and worthlessness (I guess it's the same in English, too?).
"児戯のように簡単" sounds strange for me (probably because "児戯" alone implies 簡単), and it's more often used in phrases "児戯に類する" and "児戯に等しい". I agree with @naruto in that these are somewhat classical and bookish, but I don't think 児戯 is uncommon. I expect many people to understand these phrases even in conversations, if used properly.
Usage examples from 青空文庫 :
四元の世界を眺めている彼には二元の芸術はあるいはあまりに児戯に近いかもしれない （「アインシュタイン, 寺田寅彦）
For the example regarding an exam, you can say
, which may sound a little too grandiose and novel-like but totally acceptable. (This usage emphasizes the superiority of the person for whom 試験が児戯に等しい rather than the easiness of the exam itself in my interpretation. For example if you like Death Note, this expression really suits Light Yagami.)