These words conjure up a vivid image of the Venusian children pressing close to each other as they look out of the window for any signs of sun. Bradbury's description of the children as "roses" highlights their innocence, while his likening them to "weeds" emphasizes their wild, unruly nature. Like most classes this one's clearly a mixed bag. But all the children, roses and weeds alike, have one thing in common: they're dying to see the sun.

None of the children, with the exception of Margot, have ever seen the sun, as it only comes around once every seven years, and so no one else in class is old enough to have seen it. Therefore, it's not surprising that there's so much anticipation in the air. The imminent appearance of the sun is eagerly anticipated, not least because it'll provide brief respite from the constant rainfall.

The author uses these words to build anticipation because the importance of the event they are about to witness makes the fact that Margot misses it more calamitous. By setting up the high level of anticipation that the children feel from the beginning, the author allows the reader to engage with the emotions of the children and feel the impact of their cruel prank more profoundly. 

By crowding the descriptive phrases and adjective together like he does, Bradbury gives the feeling of a jumble of pushing children simply by the sentence structure. It is a relatively common technique to string three modifiers together in a sentence. But when Bradbury increases that number to four, the reader feels the excess, as if there is not quite room in the sentence for all the descriptors, just as there is not quite room at the window for all the children to see out.