Answer:
The golden rule is to treat other as you want to be treated.
Step-by-step explanation:
By making the the grandfather feel left out and unwanted in front of the little boy, they demonstrated the actions that they seemed to think were okay. The boy figured that if the parents were okay with treating their father like that, then maybe that was how they wanted to be treated. The innocence of the boy demonstrated how powerful a monkey-see-monkey-do habit is, as the little boy was only copying what he had observed.