Why do Jem and Scout call their father by his first name?

Atticus has a special relationship with his children. Much of their time is spent with Calpurnia, as Atticus is busy working.... their mother is dead. The fact that Atticus allows his children this familiarity is rather shocking. None-the-less, as a parent, he goes out of his way to afford his children the same respect he expects from them. Possibly, Atticus' methods of speaking to and treating his children like adults sparks this form of address. We are never specifically given this information and are left to reason it out ourselves.