Answer: The boys are throwing rocks at the narrator and his father because they are Chinese.
Step-by-step explanation:
The demons in this excerpt are white Americans and they are most probably referred to as demons by the narrator because of discriminatory practices that they engage in towards Chinese people.
One such practice was shown by the little white boys throwing rocks at the narrator and his father for the mere fact that they were Chinese. Such things were common in the early 1900s when waves of Chinese immigrants tried to enter the U.S. for a better life.