In To Kill a Mockingbird, Scout and Jem call their father by his first name, Atticus. This appears rather odd to many readers, who cannot imagine calling their own parents by their first names, for it seems disrespectful or distant. In this case, however, the exact opposite is probably true. Scout and Jem refer to their father as “Atticus” out of a sense of great respect and intimacy.
Notice that the children call most adults Miss or Mr. and then their given name, Miss Maudie or Mr. Arthur, for instance. This is a sign of respect and deference. The children recognize their place as children and the adults' place as grown ups with this language. The use of the adult's first name is common while the addition of the Miss or Mr. adds a sense of propriety.
That sense of respect carries over into Scout and Jem's address of Atticus. They are not equal to their father, and they know it well. Their father, in fact, has significant authority over his children. Yet he rarely exercises it. He prefers to teach them through open discussion and questions and answers. He wants them to obey but also to understand why they must obey. Scout and Jem respect him greatly for this, for they recognize that he makes them think critically. By calling him “Atticus,” they combine respect with this recognition that they are called to grow in maturity and knowledge.
Further, the children's use of their father's first name suggests intimacy. They don't call him “Mr. Atticus” as they would if they were referring to another adult. They drop the “Mr.” out of intimacy. Scout and Jem clearly love their father very much. He is not just any “dad” or “papa” to them. They love him for who he is as a person, for they know him well thanks to his encouragement of deep discussions even about difficult subjects. They can come to him with any issue, and he will treat it with respect and seriousness. He also deals with each of his children as an individual with his or her own personality and needs, and they, in turn, recognize their father for the unique individual he is.