One of the most prominent discussions presented in the book is the idea of parenting and how to do it. Walls and her siblings were not brought up in a home that displayed "traditional" parent-child relationships and arguably, much of what they endured as children could be considered abuse or neglect. However, one idea explored through this is the question of whether Jeanette turned out "normal" despite her upbringing. Many would argue that she did. It seems, also, that she defends both her parents love for herself and her brother and sisters.
Another theme which somewhat results from the above is the notion of family loyalty. Consider the humorous and straightforward tone of the book. It is clear that though Walls is presenting memories with brutal honesty and is well aware of how her childhood will appear to the rest of the world, she also speaks with a defensive tone as if to say that she is the only one who is allowed to judge her parents based on their parenting. As a child, she is fiercely loyal throughout the story to her entire family.
Finally, you could explore the theme of poverty and survival. The children in the Walls family are nothing if not survivors. Consider the many things they do to "fit in" in their schools, to find food, and protect their odd parents from the judgemental eye of others. They are resilient, both physically and emotionally, and take care of themselves and each other.