Atticus realized that he had no choice in defending Tom Robinson. When the children ask him why he is defending him, he tells them,
"For a number of reasons. The main one is, if I didn't I couldn't hold up my head in town. I couldn't represent this county in the legislature. I couldn't even tell you or Jem not to do something again." (pg 75)
However, he realizes that it is going to be tough and the critics are going to be many. This will also affect the children. Scout has already been taunted by Cecil Jacobs and came close to fighting him over the fact that he announced in the schoolyard that,
"Scout Finch's daddy defended niggers." (pg 74)
Aunt Alexandra has been raised in Maycomb and Finch's Landing, and she understands the working of the society. She is white and therefore can give more protection to the children than Calpurnia can. She also thinks that Scout does not have enough of a feminine influence in her life. She tells Scout,
"We decided that it would be best for you to have some feminine influence. It won't be many years, Jean Louise, before you become interested in clothes and boys ---" (pg 127)
Scout is not thrilled. She has a difficult time talking to her aunt, and she feels her aunt doesn't like her. She says,
"It was plain that Aunty thought me dull in the extreme because I once heard her tell Atticus that I was sluggish." (pg 128)
When Atticus gets home, Jem and Scout both run out to meet him. When he asks if they would like Aunty to come and live with them. He tells them,
"Your aunt's doing me a favor as well as you all. I can't stay here all day with you, and the summer's going to be a hot one." (pg 128)
In calling the summer "a hot one", Atticus is making reference to the trial that is going to take place.
"I said I would like it very much, which was a lie, but one must lie under certain circunstances and at all times when one can't do anything about them." (pg 128)
Jem isn't thrilled either, but he doesn't say anything. Maycomb welcomes Aunt Alexandra with open arms and she fits right into the society. Scout says,
"Aunt Alexandra was one of the last of her kind: she had river-boat, boarding-school manners; let any moral come along and she would uphold it; she was born in the objective case; she was an incurable gossip.....She was never bored, and given the slightest chance she would exercise her royal prerogative: she would arrange, advise, caution, and warn." (pg 129)
During the Tom Robinson trial when things got heated up in Maycomb and Atticus needed help with the children, he accepted Aunt Alexandra's offer to move in. He needed her support for the summer (as is later revealed in the Missionary tea chapter) and wanted the children, particularly Scout, to be under her protection. Aunt Alexandra is a respected member of the community and Atticus may have felt people wont attack his children verbally with her around. The children, of course, were horrified.