I don't think it is really possible to argue that he is a failure in either of these areas. He certainly has to work hard to raise his two children in spite of the "help" that he receives from other individuals, but he manages to succeed in this task by raising two very headstrong individuals and helping to mould them into characters who look at the world around them and question the reality that so many people accept. He is definitely not a failure as a father in this regard.
In the same way, I think it is very difficult to argue that he is a failure as a lawyer. Even though he knew he was going to lose the case defending Tom Robinson, he does an excellent job and clearly shows that he is innocent. What the jury decided was no reflection on the skill of Atticus in any way. He fulfilled his job as a lawyer brilliantly, exposing the truth of the situation and defending his client to the best of his ability.
Atticus Finch is a character that can be described in many ways, but definitely the word "failure" is not a word that can be associated with who he is and how he acts in this wonderful novel.
Throughout the novel, Atticus is an exemplary father and a just lawyer. He teaches his children important life lessons and leads by example. Atticus is also a morally upright individual, who shows empathy for others and cares deeply about his children. Although Atticus loses his court case and makes the mistake of letting his children walk alone to the Maycomb Halloween festival, it would not be fair to label Atticus a "failure." Even though the jury convicts Tom Robinson, they deliberate for quite some time. This indicates that Atticus' arguments were enough to sway at least one jury member, which is an accomplishment in the racist community of Maycomb. Atticus' abilities as a lawyer pave the way for a more tolerant judicial system in Alabama. Atticus' mistake in thinking that Bob Ewell had got all of the anger out of his system reveals Atticus' positive outlook on life. He had no way of knowing that Bob Ewell would attack his innocent children on their walk home. Overall, Atticus successfully raises two competent, sympathetic children, who are considered morally upright individuals with perspective.