This is an interesting question. The children bridge the gap between the innocence, simplicity and more happy and serene world that children live in, and the bitter, racist, hateful world that adults live in. For example, it is because of the children that we are able to see Boo's true and kind self. The entire world thinks he is some kind of "malevolent phantom" that steals, commits crimes, and haunts the town. But, through Jem and Scout (who are children), we are able to see that he is just a lonely man who desires friendship, and is able to commit great and heroic acts of good for the town. The kids were the bridge that allowed us to cross over and see him for his true self. Also, consider the mob outside of the jailhouse. That is fully within the realm of the adult world, and racism, hatred and violence are the ruling factors of that world. Those men came to do harm to a man they hated because of his race. But, as Scout and Jem come onto the scene, Scout creates a bridge from that world to the more simple and kind world of childhood perspectives. She addresses Mr. Cunningham by name and starts talking to him about his life. Through this act, he is able to see how ridiculous he must appear to her, and he is unable to stay fully within his vicious, adult world anymore. Scout bridged the gap between violence and childlike acceptance, and helped the mob to realize what they were doing was over the top.
Children are a bridge that allows people to cross over into more calm, loving, simple, and good behaviors. Because of the children, we see who Boo really is, and racism is exposed for the ugly hypocrisy that it is. I hope that those thoughts help a bit; good luck!