it contributes to the story by showing that dad wants everyone to follow the guide and to not joke around.
Reason: based on what I see the dad hushes them after joking about something and to follow the guide. Based on context clues it seems that they joked about not following the guide but dad wants the to stop joking about it and to follow the guide.
if you post this question again please add the rest of the text. And take this answer with a grain of salt as it's hard to answer without seeing the rest of the text.
