schoolyard

play, amuse oneself, enjoy oneself / spend one's time idle, do nothing / for an item not to be in use, be spare or idle

child

child's naive mind, child's juvenile mind

plural of 子供: children

(offensive, Internet slang) a man whose age is greater than about thirty or forty who lives in one's child room and with one's parent(s).

(figuratively) being carefree and unaware of an imminent danger