The authors purpose would be to evoke the feeling of being alone without any one to rely on.
In this poem, the author's purpose is most likely to convey the feeling of desperation that the child feels when left alone. We learn that he is very upset to see his father walk fast. We also learn that he fears being lost if the father does not slow down for him. This poem shows the importance that the father has on the life of the boy, and how desperate people can become when they fear loneliness.