We could also describe this as situational irony. Irony occurs when a discrepancy exists between what we expect and what actually happens, and situational irony occurs when events arise that defy expectation. When the narrator describes the children, recently out of school for the summer, playing "boisterous[ly]" and "talking among themselves," we could hardly anticipate that they are picking "the smoothest and roundest stones" so that they can participate in the violent and horrific stoning of an innocent, if outspoken, woman. We tend to think of children as innocent, and the description of them in the second paragraph does nothing to change our expectation of their innocence. Therefore, when we learn later that they are, essentially, little murderers who are complicit in and even excited about the public, ritualistic stoning of one of their neighbors, this certainly defies our expectation.

The device used in this instance is foreshadowing. Foreshadowing is where the author gives us a small hint of what is to come, without entirely giving away the outcome of the situation. In "The Lottery," the children initially are gathering the stones in apparent innocence as they enjoy the beautiful weather and the end of their school day. As the story progresses and the lottery ensues, the reader becomes aware that the stones serve a much more nefarious purpose. The children were actually gathering stones in order to stone an innocent citizen to death. 

By including the reference to the stones in the beginning of the story, the author creates an atmosphere of curiosity and innocence, as the reader wonders what the stones are for and likely believes they are for some childish game. When it becomes apparent that the stones are for murdering a person, the juxtaposition of the two different uses is highlighted.