Great question! Your answer to this story is going to be based on what you think the primary point of the story is. Do you think that this is primarily a ghost story of the supernatural which focuses on the spooky relationship between Paul and the rocking horse that enables Paul to eerily predict winning horses or is this primarily a moral tale which borders on the satirical as Lawrence drives home didactically his message about the dangers of materialism and how it can rip families apart and even lead to death?
Based on your answer to this question you can say that the tone of the story is either ominous and mystical, or satirical. If you go for the former you can focus on passages that describe how Paul describes the further you get into the story, particularly on his voice, his eyes and the rocking of the rocking horse. If satirical is your answer then you can talk about the greed of the mother and the irony at the end when she has gained her dream but lost her son in the process.