what does attitcus mean when he says it seems that only children weep

When Jem asks how the jury could have convicted Tom Robinson, Atticus says, "I don't know, but they did it. They've done it before and they did it tonight and they'll do it again and when they do - seems that only children weep."

By this, he's referring to the way the jury convicted Tom Robinson even against the good evidence they'd collected. Basically, it amounted to the fact that the jury was willing to believe a dishonest, trashy white man over a black man, simply because Tom Robinson was black. Atticus is saying that when such terrible acts of racism happen, it seems that children like Jem are the only people who care.

There seems to be something to do with the innocence of children, who don't understand the cruelty of the world, too.