The key themes of An American Childhood largely deal with the differences in the way that children and adults view the world and seek happiness in it.
The main theme has to do with the consciousness of the narrative; the author is self-aware of the changes, both controlled and wild, in her personality and tastes as she is exposed to new things and ages. This theme could be summarized as, "The world is revealed to us in small steps, through details, and is always changing." We could also characterize some of her observations about adults based on their response, or lack of it, to the world; "Limiting your exposure to the world probably won't make you happy". This is accentuated as the narrator matures in part 3, where she becomes more focused on material things but is nevertheless unhappy for the first time in her life. There are exceptions to this, such as her mother, but in general there is a sense that adulthood typically means unconsciously throwing away one's happiness amid the pursuit of more socially defined standards of success and belonging.
Some other themes might include;
-Many adults have forgotten how to be curious about or impressed. The narrator's retention of this curiosity, and her desire to go to college, are part of what helps keep her from becoming too unhinged and rebellious during her teenage years.
-Consciousness is not linear or bound to "stages". In fact some of the elements that the narrator reflects on (such as the snowball, linked below) demonstrate a combination of consciousness, unconsciousness and what we as adults might perceive as merely a justification of bad behavior. However in the context of the novel as an account of a growing mind, all of this makes much more sense. Our consciousness does not grow according to any predetermined or external standard.