What are the literary devices in this poem?

alliteration, much of which in this case is sibilance (repeated "s" sounds), plus parallelism (shows son and daughter being different in action, but the same in tone/reflection), and reference (mirror, mirror, refers to Snow White's step mother saying "mirror, mirror, on the wall..." and implying "fairness of form or visage"). There might be more, but it would be a stretch...the poem is quite a simple one, although effective.