There are a lot of great points raised by this novel, in the plot, in the character development, and in the style. But one of the most important ideas in the book is just that Ellie doesn't have to be a grown up to be a hero. She is a hero by standing up for her family and her brother, even though her parents are arguably defunct by the stress of their post-apocalyptic life. She acts not only for her brother's protection and well-being, but for her own, using a self-love and protection that she doesn't really have at home.
Her parents aren't thrilled to see her, not because they hate her or anything, but because it is confusing to them, and it isn't like they are hanging out at a picnic when she shows up. They are in survival mode too. The people have been (falsely) informed about a toxic yellow gas that the wall around London is designed to "protect them from." Meanwhile, powerful men become more powerful by exploiting the weak.
Ellie and her brother end up in a difficult situation, having to escape London into certain death. When they learn that there was never any yellow gas, that makes them feel better, but Ellie and her brother talk about their parents. Without a way to save their parents, they find a strange kind of sadness that is very important: They realize that there is nothing they can do to fix the world or to rescue the lives of their parents. All they can do is be in community together, finding a way to survive.
