"[be] (like|) child's play" の定義と意味

What is the origin of the idiom "be child's play" and when to use it?

The idiom "be child's play" is said to describe a task or activity that is very easy and simple to accomplish, as if it were something a child could do without difficulty. The origin of this phrase is straightforward; it likens an easy task to the kind of play that children engage in effortlessly. The idiom has been used in the English language for a long time, with documented usage dating back to the 17th century.