What does it mean when you dream of causing a car accident where a baby dies and you cry in your sleep because you killed the baby?

The persons and objects that appear in dreams are usually symbols that represent something else. A "child" often represents something produced by the dreamer. For example, in a writer's dream, a child would be a symbol of the writer's book. A dream of murdering one's own child would mean the dreamer had destroyed something the dreamer had lovingly created.