Rain arrives in this poem one sound at a time. First come the "Drip! Drip!" of drops, then the "Splash! Splash!" as they hit water. The words are short and the rhythm is simple, so you can almost hear the rain tapping out the lines.
Then the focus shifts from the weather to what it leaves behind. Puddles gather "here and there," and so do paper boats, and the repeated phrase makes the whole scene feel scattered and lively. At the end we finally see who is behind the boats: children. The poem repeats that last line, "Children sailing paper boats," as if it wants to linger on the image.
The speaker never states a feeling, but the mood is clearly cheerful and playful. Rain is not treated as a gloomy interruption. It becomes a chance for simple fun, and the poem suggests that joy can come from very small things, like a puddle and a folded piece of paper.