The above situation can best be described as situational irony. In the story "TheLottery", it appeared at the onset that the children were gathering stones for fun, but their intentions were much more sinister.
<h3>What is situational Irony?</h3>
Situational irony in literature is a condition in which actions create an opposite effect or take on a contrarytrajectory other than those that it was originally intended to have.