In "To Kill a Mockingbird", Atticus is a good father because he tells his kids the truth. Where are clues?
In Chapter 9 of this book, Atticus clearly lays out his belief on children and the truth. He is reacting to criticism that he should not discuss the upcoming case with his children, that they are not old enough to hear and understand such a crime as rape. This is his response:
When a child asks you something, answer him, for goodness' sake. But don't make a production of it. Children are children, but they can spot an evasion quicker than adults, and evasion simply muddles 'em.
Atticus is saying that you simply address what the child asks without turning the moment into a lecture. He is also pointing out how observant children are, showing that his policy on truth comes from the respect he has for his own children.
Times when he shows this attitude towards truth start early on. He admits to Scout that her teacher was wrong to criticize her for knowing how to read, but explains that as a teacher, she must be followed. He could have left out the first part, but he doesn't want to lie to Scout.
During the trial, Atticus doesn't hold back from the kids. He allows them to stay in the courtroom once they are discovered there because he understands that they need to "see" the truth. He also is straight-forward about his decision to defend Tom:
If I didn't I couldn't hold up my head in town, I couldn't represent my county in legislature, I couldn't even tell you or Jem not to do something again.