"Metaphor" is commonly defined as a word or phrase used in a manner for which it is not normally applicable, but for which a parallel is identified. Ed Reed's poem of youthful wonder and lost innocence is replete with metaphors. In fact, the poem's title "The Key of the Kingdom," is itself a metaphor, possibly borrowed from the Biblical phrase from Mark 16:19 (I will give you the keys to the kingdom . . ."), referring to, as in the poem's phrase "Only children had the key, The key of the kingdom," to the wonderous stage of life in which mystery and imagination are given free rein to conjure whatever images seem appropriate at the time and in whatever way the mind of a child desires.
Another metaphor in "The Key of the Kingdom" is in the phrase "we have lost the key," which in normal conversation refers, obviously, to the literal sense of having lost the instrument by which one can enter a structure. In the context of his poem, Reed uses the phrase to mean "we are no longer children and, consequently, no longer enjoy the carefree lives of our youth when the world existed to be explored. We are now adults with responsibilities." Coupled to that phrase is another metaphor, "And it has perished with the rust of misuse." Again, literal application of the phrase would refer to an object, particularly a mechanical object, that atrophes from having sat unused for a long period of time, for example, a motor (which can be a metaphor for the brain, in which the imagination exists).
Finally, the poem's final stanza, "Age is the grave yard/Of all our youthful hopes," is a metaphor in that "grave yard," a tangible object associated with death, is in this case used to describe the demise of one's "youthful hopes," or dreams and ambitions. "The Key of the Kingdom" begins as a reminiscence regarding the idealism of childhood, when summer vacations are spent exploring the woods and allowing imaginations to conjure up images of fantastic objects, both terrifying and wonderous. It evolves, however, into a mournful contemplation of the toll on one's psyche of the inevitable process of aging.