to suggest that children consider the lottery a game and do not understand it
Explanation:
According to the passage from "The Lottery", the narrator describes school being over for the summer and how uneasy the children were feeling about their liberty.
He describes Bobby Martins and some other boys picking up the smoothest and roundest stones and gathering them in a pile and guarding them.
Therefore, the most likely purpose of depicting how the boys are collecting the stones is that to suggest that children consider the lottery a game and do not understand it