the mind > attention and judgement > importance > unimportance > [noun] > that which is unimportant > insubstantial > childish (2)
As a count noun: an instance, embodiment, or display of childishness, immaturity, or triviality; (in later use also spec. in plural) juvenilia.
Play or activity befitting a boy; (hence) something trivial or easy to accomplish; = child's play, n. Also: an instance of this.