As was mentioned in the previous post, Chapters 10 and 11 are important turning points in the lives of the children and the plot of the novel. Both Jem and Scout learn valuable lessons after spending time with Mrs. Dubose and witnessing their father shoot the rabid dog. Atticus also teaches his children what real courage looks like after he elaborates on Mrs. Dubose's battle with her morphine addiction. The first part of the novel primarily focuses on the children's infatuation with Boo Radley and portrays Atticus giving his children valuable life lessons. The second part of the novel focuses more on Tom Robinson's trial and the children's maturation. In the second part of the story, Jem and Scout witness racial injustice firsthand, which deeply impacts their perspectives on life. Both children also apply their father's previous life lessons and develop into morally upright individuals. Harper Lee chose to end Part I after Chapter 12 in order to shift the plot toward the Tom Robinson trial and to explore the children's moral development. 

Chapters 10 and 11 of To Kill a Mockingbird prove to be turning points in the lives of Atticus' children. In Chapter 10, Jem and Scout discover that Atticus is not "feeble"; he has two special skills--marksmanship and humility. Chapter 11 is particularly important to Jem's approaching adolescence and maturity. He learns several lessons from his stay with Mrs. Dubose--that people are not always what they seem and that an act of seeming drudgery can have positive implications--and also has to deal with the death of someone who he has come to know.

The second part of the book begins with Jem's growth into young manhood and how Scout must deal with her brother's changes. It also detours from their infatuations with Boo Radley to the second major plot of the story--the Tom Robinson trial.

I agree with the above post. The episode involving Mrs. Dubose is the last experience Scout and Jem will share as children. Everything after this (particularly the trial) will be observed, felt, and analyzed differently between the two. Scout maintains her innocence, and a major conflict in the later parts of the book will be her attempt to understand why Jem thinks/acts the way he does. Jem, on the other hand, has little time for Scout, and he processes each new development in the trial quite personally.

By ending the first part with the Mrs. Dubose episode, Lee is also introducing an important lesson to the children. Think of it as a last attempt to fortify their defenses before they are forced into situations beyond their years. Prior to this, Atticus often tells Scout to keep her head when times get rough, and Mrs. Dubose seems to be the testing ground before they face the rest of the community. Thus, they have one final instruction in courage and grace before they have to actually practice those values.

In my opinion, part one of the book focuses on the childish world of the Scout, Jem, and Dill. Their main focus and concern is with playing children’s games such as the make believe plays about the Radley’s.

The second part of the book focuses more on their exposure to an adult world as the novel shifts its focus from the Radleys to the Tom Robinson trial. The children are exposed to real world problems and lose interest in the childlike world of make believe. We later see they are not immune from the dangers of the adult world as they are thrown into a world of hatred and racism. .

Chapters Ten and Eleven represent the end of Jem's and Scout's keen interest in the Radley house and specifically in Boo.  Lee chooses to the divide the novel into two plots which she ties together at the end.  Chapters Ten and Eleven temporarily end the first plot (Boo Radley), but Lee adeptly uses the book's first section to foreshadow the second section (Tom Robinson's trial).  Boo and Tom are the novel's mockingbirds--they do good for others and mean no harm, but others, through misunderstanding and prejudice "harm" both characters.

The novel's first part also ends Jem's and Scout's innocence. As they obsess over Boo's "dark" past and present, they play childish games with Dill and engage in imaginative schemes to get Boo to come out of his house.  As the trial begins, Jem and Scout endure harsh comments from Maycomb's residents, realize that not everything in life is just, and begin to view their father as a hero.