Just to have the correct answers.
He says the mob of men were simply ignorant. Atticus says it took the simplicity and honesty of a child to appeal to the humanity within them. He thinks that children have a way of diffusing tension and a way of seeing the world without prejudice (until they learn it).
When Atticus says this, he means that a child has a way of seeing the world through the eyes of innocence. Without knowing the harms that come with prejudice, Scout has this repertoire to assuage the seriousness and tension between the ignorant mob of men that any adult would not have.
