This is a good question. There are several signs that show that all is not well. First of all, right from the beginning, Jackson says that the children had a feeling of uneasiness. This is particularly odd, because they are off school for summer break. She writes:
School was recently over for the summer, and the feeling of liberty sat uneasily on most of them; they tended to gather together quietly for a while before they broke into boisterous play.
Then there is the odd act of the boys collecting rocks. Moreover, there is a pile of rocks on the corner. Jackson will not let the reader miss this pile, as she mentions it a few times.
Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones; Bobby and Harry Jones and Dickie Delacroix-- the villagers pronounced this name "Dellacroy"--eventually made a great pile of stones in one corner of the square and guarded it against the raids of the other boys.
A few paragraphs later, she mentions the rocks again.
Bobby Martin ducked under his mother's grasping hand and ran, laughing, back to the pile of stones.
Finally, there is a mysterious black box, which Mr. Summer carries. The reader is left wondering what is in the box or what it is used for.
When he arrived in the square, carrying the black wooden box, there was a murmur of conversation among the villagers, and he waved and called.
The stones and the box together with the feeling of uneasiness show that the tone is ominous irrespective of the idyllic setting.
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