The poem is only three lines long, but it packs in a whole scene. The speaker looks back at a childhood tantrum, a "fit," and describes it as "a one-minute storm." That comparison does a lot of work. A storm is loud and overwhelming, yet it passes quickly, and the same is true of a child's rage. Seen from the distance of adulthood, it shrinks to something brief and almost harmless.
The odd word is "diligent." We don't usually think of a tantrum as hard work, but a child in the middle of one throws everything into it, with total commitment. The speaker seems to look back on that effort with a mix of amusement and tenderness.
The last line, "that almost could be art," adds a playful twist. The fit was so intense and so fully performed that it came close to being a kind of performance. The mood is light, wry, and affectionate, with a little wonder at how seriously children feel things.