This is a fantastic question. The interesting thing about this question is that the book never really addresses why the children call their father Atticus instead of "dad" or "father." In light of this, we can only guess.
First, it should be noted that when Scout reflects upon her past, she does call Atticus father. Here is an example:
"Jem and I found our father satisfactory: he played with us, read to us, and treated us with courteous detachment."
Second, some might say that this practice was owing to the death of their mother. In other words, when Jem and Scout's mother passed away, Atticus was not able to raise his children well. Aunt Alexandra seems to suggest this. For example, she wanted to educate Scout in proper manners.
Third, another thought is that Atticus wanted his children to call him Atticus. I prefer this interpretation, because there does not seem to be a problem at all. The children call their father by the his first name and everything seems natural. This, then, begs the question why Atticus would want this. In my opinion, Atticus was trying to teach his children that he valued their opinion as if they were adults.
This question has already been asked/answered here on eNotes. Here is a link for you: http://www.enotes.com/to-kill-a-mockingbird/q-and-a/why-dont-jem-scout-call-atticus-father-dad-instead-17955
Whilst there are a few possible reasons we were told that it is because Harper Lee gets Jem and Scout to call Atticus "Atticus" instead of Dad because it symbolises the special and unique relationship that the children have with their father and how this relationship is quite different to that of a normal father/son(daughter)
Hope this helps
Because she is taught to talk like and act like an adult.