I personally think it’s because Atticus treats them like their mature and older, and he makes them feel like they are on the same level of understanding as him, so that makes them feel comfortable calling him that.

– Delisa Grant

I agree with Delisa but I would like to add that it’s like calling a teacher by their first name instead of Mr. or Mrs. If the teacher allows you to call them by their first name it would imply that you are on more casual terms with them. Calling parents by Mom or Dad implys that they are above you at all times, but if they let you call them by their first name you would feel that you are on the same level of authority, even if in reality parents being parents will still “control you”.

-Eli Papanicolaou

I agree Eli and Delisa, it is because Atticus and the children treat each other like equals. I also felt in the novel that the children had a very formal relationship with Atticus and that may have something to do with it as well.

– Jacob Knott

I completely agree with everything thing that has been said. I also feel that Jem and Scout call Atticus by his name instead of father because he treats them with a great deal of respect. Throughout the book it becomes clear that Atticus does not agree with everything that occurs within Maycomb County and does not expect his children to conform to Maycomb’s ‘snobby’ ways. I  feel that Harper Lee made his characters Jem and Jean Louise call their father Atticus as a way of showing how different they are as a family from the other residents of Maycomb county. The small part of the Finch family basically does what they please within a respectful extent, by Atticus’s children calling him by his first name it adds to the idea that they will not conform to any of the ‘proper’ ideas within Maycomb.

– Jessica Massaad