Jermielee describes a small boy who turns the speaker's day upside down, and the poem is full of energy because of it. The cousin pounces like a hunter, bounces on the bed like a kangaroo, and shakes the speaker awake like a tambourine. The comparisons are funny and a bit over the top, which matches how a five-year-old seems to fill up a whole room. Even the bed "screams for mercy."
The most amusing moment comes when the cartoons start. The wild kid who was just jumping and dragging people around suddenly goes "as silent as a mice." That sudden switch shows how quickly a small child can change.
The speaker sounds worn out but amused, and the final line, "I'm so glad he is my cousin," shows what they really feel. All the rough play adds up to affection. The mood is playful, noisy, and warm, like a lively morning with someone you love.