An idiom is a phrase which is widely understood to have a meaning other than its literal one. Every language and society has idioms: in English, we might say that someone has "cold feet" if they are frightened to commit to something; in German, "kalt kaffee" (cold coffee) is something which has already happened and cannot now be changed.
There aren't any idioms in this short story by Ray Bradbury which are recognizable as English idioms, even though the story is in English. There are, however, idiomatic phrases and images which we can assume to belong to the Venus-based society in which the children in the story are living. Because they are living in a world where the sun comes out only once in seven years, there are understandably idioms about the sun and about the constant rain. For example, the children imagine a "coin large enough to buy the world with," which we understand to mean the sun. The rain dropping on the roof is described as "clear bead necklaces."
Not being part of this society, it's difficult for us as readers to determine whether these images are simply metaphors, or if they are idiomatic. Idioms are metaphors, after all, but they are commonly used and generally expected to have a certain meaning. We know that the children of this society think of the sun as being like a lemon; we can also assume that because more than one of them imagines the sun as "a coin large enough to buy the world with," this is an example of idiom or shared metaphor.