In "To Kill a Mockingbitd," why does Atticus think it acceptable for the children to be at the trial?characters

Even though Atticus has not intended for the children to be present at the trial, he probably reconciles their continuing to attend since they have already hear much of the trial, and also since he has not tried to shelter them from real life, anyway.  For instance, at the end of Chapter 9, as Atticus talks with his brother about his assignment to defend Tom Robinson, he expresses no optimistic expectation of the outcome, yet he does express the hope that he

can get Jem and Scout through it [the trial] without bitterness, and most of all, without their catching Maycomb's usual disease.

So, true to his character as demonstrated in earlier in the novel, Atticus allows the children to remain in the courtroom, trusting as he always has in their reasoning abilities.  He knows that if Scout and Jem reach an ethical conclustion on their own, they will abide by this conclusion all their lives, just as he, too, abides by his ethical conclusions. 

Atticus hopes that by listening to the trial of Tom Robinson, the children will learn the meaning of Mr. Raymond's words to them in Chapter 20 of "To Kill a Mockingbird": "Cry about the simple hell people give other people--without even thinking."