What is the "nightmare" that now descends upon the children?
The nightmare unfolding around the children’s safe and innocent life is the racism the children will face throughout the arrest and trial of Tom Robinson. Scout and Jem first confront the racism of the town when they follow Atticus to the jail. Atticus’ intention is to protect Tom from a lynch mob, and when the children arrive at the jailhouse, the mob is threatening and angry. Scout, in her innocence, points out Mr. Cunningham in the mob. Mr. Cunningham represents the racism present in the average Maycomb resident, and this scene allows Jem and Scout to see that this hatred is predominant in people they know and the everyday events of Maycomb.
Throughout the rest of the novel, Scout learns about hypocrisy through the teachings of her teacher about Hitler’s discrimination of Jews and the outlandish hypocrisy of the missionary circle raising money for a tribe in Africa but who won’t help their neighboring black community. She also learns about Dolphus Raymond and the discrimination he must endure to live the way he wants. Jem and Scout are also harassed by their classmates at school for Atticus defending Tom. Even Mrs. Dubose yells at the children and calls their father a derogatory name.
The children are living in a nightmare because they are being ostracized and bullied by their friends and acquaintances. They are no longer living the dream-like state of children but are living the reality of how racism can affect them and those around them.
To Kill a Mockingbird, is one of the most beloved works of literature. Harper Lee has such a way of putting you right there in the middle of the story.
There are a few different meanings of what the nightmare is to the children. In this part of the book, several things are taking place. The trial of Tom Robinson is all the town can think or talk about. The people of Maycomb are so ready to find the man guilty. Atticus is trying hard to keep all of the hatred away from Jem and Scout. When the mob approaches the jail, Jem, Scout and Dill are afraid for Atticus. This is the first time they had seen their neighbors act this way. It is also a nightmare for them to learn the attitude of the people against a black man. Jem and Scout are losing the innocence they once took for granted. The other nightmare for the two of them is what they have to endure with Mr. Ewell. Their lives are almost taken away from them, if it weren't for Boo.
The nightmare for Jem and Scout, unfortunately is reality. Atticus can't save them from the harsh realities of the world. All he can do is be the best influence on them and teach them how to treat people, which is exactly what he does.
Just to add a couple thoughts to the previous answer, the nightmare could also be how the family is now thrust front and center into the debate surrounding the trail. Since Atticus is flying in face of Maycomb's normal social conventions, simply lynching Tom Robinson and attaining mob justice, he is bringing down all kinds of trouble on to his family. This can range from simple insults and dirty looks all the way up to the revenge Bob Ewell seeks on Atticus and his children.
The "nightmare that was upon us" is how Scout describes, in Ch. 15, the events that led up to Atticus' confrontation with the townsmen while he was protecting Tom in jail. For the first time, Scout and Jem witness the hatred of the mob and the horrors of racism, and see their father in a vulnerable, and perhaps deadly, position.
This is when the children see the hatred held within the town and the scary mobs that the others do protesting the case Atticus takes up, it is scaring them. They never realized how scary and hateful the townspeople actually were, but seeing this new form helped them realize the hatred and racial prejudices concealed with the town.
the nightmare as i would say that is upon the children is that the mob scares and frightens them as they see atticus defend tom robison