Before diving in, a quick recap of "Hansel and Gretel," the Grimm fairy tale that the poem is playing with: Hansel and Gretel live with their poor woodcutter father and stepmother. Insisting that the children are too costly to feed, the stepmother convinces the woodcutter to abandon them in the forest. Later, lost and hungry, the siblings come across a house made of gingerbread and other sweets. They're lured inside and then imprisoned by the owner of the house, a witch who intends to fatten Hansel up and eat him. Before she can follow through on this wicked plan, however, Gretel outwits the witch and pushes her into a fiery oven. The witch burns up, the stepmother randomly dies, and Hansel and Gretel live happily ever after.
Like many fairy tales, it's a dark story when you really think about it—filled with poverty, child abuse, cannibalism, and horrific death via oven. "Gretel in Darkness" latches onto this, well, darkness, and opens with unease. "This is the world we wanted," full stop, could be the beginning of a poem about fulfilled wishes—a happily-ever-after poem. But already the reader has a sense that this wanted world isn't all it was cracked up to be.
Part of the clue is in the past tense: the world "we wanted" is not the same as the world "we want." Another hint is in the use of the end-stopped line, which brings the thought to a swift, dull close. It's worth remembering the title of the poem, too: the reader knows that the speaker is the Gretel of fairy tale fame. "Darkness" doesn't seem to be suiting her.
The unease of this first line only deepens in lines 2 and 3, where Gretel elaborates on the nature of this wanted world she lives in. "All who would have seen us dead / are dead" is hardly a joyful celebration of life: Gretel's world is permeated with death, and it seems it's kill or be killed here. The enjambment of this sentence drives this point home. Line 2 could be the beginning of a very different kind of sentence, maybe "All who would have seen us dead... realized the error of their ways, apologized, and moved to a different forest!" But again, grim reality lands with a thump in line 3 with the repetition of the word "dead." Enjambment and diacope set this unpleasant surprise up like a dark joke.
Speaking of set-ups: there's a hint of menace in the strong alliteration on /w/ sounds here ("world we wanted"), which will come to a climax in the rest of line 3. All of those /w/ sounds build up to the arrival of a nasty character: the witch.