The poem's metaphors illustrate how the speaker saw and experienced the world as a child. They demonstrate both her vivid, active imagination and her incomplete understanding of subjects like death.
For example, the speaker twice compares a marble tabletop—where a "stuffed loon" sits—to a "white, frozen lake." This is a fitting and thought-provoking metaphor on several levels. Loons are water birds, so they do nest on lakes and ponds. And the poem is set during a cold winter in Nova Scotia, so the surrounding lakes would certainly be frozen. It's apt and creative, then, for the speaker to compare the loon's tabletop to an icy lake; after all, both surfaces are cold, round, and white.
At the same time, loons don't live on frozen lakes; they migrate south in the winter. So there's something unsettling about the idea of a motionless loon on a frozen lake, just as there's something unsettling about a loon standing on a parlor table. The loon is there in the house because it's been "shot," "stuffed," and turned into a decoration. But to the young observer, who doesn't yet understand death, it still seems quasi-alive, as if the table might somehow be a version of its natural habitat.
The speaker also compares "Arthur's coffin" to "a little frosted cake" (lines 27-28). In other words, it's smallish (not elongated like an adult coffin), decorated (with flowers and/or carvings in the wood), and perhaps white, like cake icing. To her, the coffin looks appealing—a sign of her confusion about what death really represents.
The simile and extended metaphor involving "Jack Frost" (lines 32-40) further illustrate her struggle to wrap her mind around death. She doesn't fully grasp why Arthur looks "all white" except for the red of his hair—that is, she doesn't understand pallor mortis, or the paleness of corpses. So she invents her own explanation for Arthur's pallor: she compares him to a "doll," then imagines that "Jack Frost," who paints the leaves of autumn, tried painting Arthur red but quickly gave up.