In the beginning of the story, "the children pressed to each other like so many roses, so many weeds, intermixed, peering out for a look at the...

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.