The speaker's one wish is to go back to being a small child, even if only for a day. Childhood, in their memory, was a time when the world felt enormous and exciting, "one big burst of color / ready to splatter itself on a page." Being small meant being amazed rather than afraid.
Now that they're older, the speaker feels almost the opposite. Growing up hasn't made the world easier to handle; instead, "the weight of the world / could crush me at any moment." The contrast at the heart of the poem is striking: as a child, they felt they could "crumble" the world in their dirt-covered hands, powerful and fearless, while as an adult they feel fragile.
The mood is wistful and a little sad. The repeated line "to be this small again" sounds like a quiet plea, and the dash and halting ending make the longing feel unfinished, as if the speaker knows the wish can't come true.