How is Dill a coward in the first eleven chapters of the book?

Charles “Dill” Harris is a boy of ten or eleven during most of the period the novel covers. He is a confused, insecure person, largely because his home life is unstable and he is sent away to Maycomb every summer.
“Coward” seems a harsh label to apply to such a young child. The events during the period that Harper Lee addresses in chapters 1–11 offer relatively few occasions to show bravery or fear, but Dill shows some of both. His insecurity makes him prone to invent stories. Rather than a coward, it might make sense to evaluate times when his fear affects him negatively, when it keeps him from accepting responsibility or prompts him to lie.
Dill’s inventions with regard to Arthur “Boo” Radley also show him sometimes to be inconsiderate or unkind (chapter 4). Considering their activities just a game, he apparently does not think of Radley as another person whose problems might be worse than Dill’s problems. He also encourages Jem to take more risks, perhaps thinking of Jem as being braver than he is.
After the children trespass on the Radley property to spy on their neighbors, Nathan Radley fires his shotgun but does not hit them (chapter 6). Jem leaves his snagged pants on the fence, however, and is afraid to tell his father the truth. Dill shows loyalty to his friend by not telling Atticus what really happened, but that is not a courageous stance.

In the first eleven chapters of the novel, Dill is portrayed as a coward because he fears Jem and Scout's reclusive neighbor Arthur "Boo" Radley. Like many children in the small town of Maycomb, Dill believes the rumors and legends surrounding Boo Radley and believes that he is a "malevolent phantom." Despite Dill's fear of Boo, he is fascinated by Boo and curious to get a good look at him. Dill attempts to disguise his fear by cowardly daring Jem to knock on Boo's door and place a letter on Boo's window sill. Rather than run up to the Radley house on his own and knock on the door, Dill prefers to keep a safe distance away from the Radley home and is completely comfortable with letting Jem assume the risk. One could also consider Dill cowardly for neglecting Scout and running away from her to avoid getting beat up. Other than not volunteering to assume the risk of entering Boo's yard alone and purposefully avoiding Scout, Dill is not much of a coward.