Maybe it's a fantasy story where the rain has magical properties and the kids discover its secrets. The plot could involve them using the rain's power for something extraordinary.
It could be an adventure story where the kids have to figure out how to survive and thrive in the constant rain.
It might be a coming-of-age story. The kids could face challenges and grow because of the unique environment of the rainy planet. They could learn valuable life lessons and develop their characters through these experiences.