Can the children go outside every day in the rain or can they only go when the sun comes out every seven years?

While the text does not explicitly answer this question, there is evidence which suggests the children never or very rarely go outside, making the hour-long appearance of the sun that much more spectacular.
Early in the text, Bradbury writes that "A thousand forests had been crushed under the rain and grown up a thousand times to be crushed again." The planet also experiences a "concussion of storms so heavy they were tidal waves come over the islands." These clues indicate that the climate of the planet is not merely rainy, it is full of volatile storms which would be dangerous for children to play in.
Later in the story, we learn that Margot "would play no games with [the other children] in the echoing tunnels of the underground city." We can infer that the children play in the tunnels of the underground complex in which they live, rather than outside. 
Finally, when the children do go out to play, they go straight into the natural growth. They bound into "a nest of octopi, clustering up great arms of fleshlike weed, wavering, flowering in this brief spring." If the children did typically go outside, it stands to reason there would be a playground or designated area of some kind for them to be in. This, however, is not the case. It would seem they never go outside, and, consequently, they make a beeline right for the jungle's overgrowth simply because there is nowhere else to go.