why does will and Jim hide under the street grille

At Ned's Night Stop, Charles sits and drinks coffee. Mr. Dark sits next to him and announces to Ned that he is looking for two boys. Hearing this, Charles leaves and walks to the library, pausing to buy a cigar. Will and Jim spot him but Will tells Jim not to call to him. They hide under a street grille. Charles lights the cigar and drops the cigar band into the drain where they are hiding. His eyes follow it and he sees the two boys. Just then, Mr. Dark walks out of the bar toward Charles. Will tells his father to look away from the grille and towards the courthouse clock.