The line in question reads: "The children had stones already, and someone gave little Davy Hutchinson a few pebbles."
It is significant because little Davy must take part in the stoning of his mother, Tessie. By doing so, the townsfolk make sure that even the littlest, seemingly most innocent member of the bloody ritual has some culpability. It is not that his stones will harm his mother (others will see to that), but that he shares responsibility.
We know Davy contributes to actions he does not understand in the least. Consider the lines where he is choosing his paper from the lottery box:
"Harry, you help little Dave. Mr. Graves took the hand of the little boy, who came willingly with him up to the box. "Take a paper out of the box, Davy," Mr. Summers said. Davy put his hands in the box and laughed. "Take just one paper," Mr Summers said. "Harry you hold it for him." Mr. Graves took the child's hand and removed the folded paper from the tight fist and held it while little Dave stood next to him and looked up at him wonderingly."