There are several literary devices in this poem.
The first and most often used is imagery. The poem includes many images: "sharpening pencils," "little desks," etc.
There are a number of metaphors in the poem, and these give the poem more of its power. Lines like "waves of breath" and "wall of baby" are vivid, and they express the affection the speaker has for the baby well.
Repetition is used in a more subtle way. The connection between the "waves of breath" and the wave the child gives later in the poem is quiet, but it creates a unity in the poem, linking infancy to childhood.
Sections of the poem create an ongoing analogy: the section that begins "forest of juice boxes" gives the impression that school is a place of wildness, that the child is threatened somehow by growing up.