What effects is the author trying to produce when he places the blowing of the mill whistle and the discussion of the mill in the middle of the Gibbs' children's morning?

Likely, the family plans its day around the blowing of the whistle. When the whistle blows, the people in the town know that it's time to get moving and get going. Work is beginning, and school is just about ready to start. The mill is the town's biggest employer.