This statement is made by Atticus to his brother, Jack, in Chapter 9 of the novel. It is in response to Jack's admission that he changed the subject when Scout asked him the difficult question "What's a whore-lady?" Atticus is a straight shooter when it comes to answering his children's questions, and he always tells them the truth. He knows that
"... children can spot an evasion quicker than adults, and evasion simply muddles 'em." (Chapter 9)
Atticus also knows that Scout is not-so-secretly listening in on the conversation with his brother, and Atticus wants her to hear what he has to say because
"I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough..." (Chapter 9)
It is clear that Atticus's strategy worked, since even in his old age, Jem and Scout still "consulted Atticus" when they needed to "settle an argument."