Broadly speaking, this novel is about suffering. Because the main character is a child, the novel removes the question of guilt from the equation, and what's left is a heartbreaking depiction of the tragedy of human life, and the senselessness of our suffering. By making the villain of the novel the child's father, Marina is given the extra challenge of learning to forgive the father who hurt her. In a way, the novel is about the supreme integrity of Marina, but in another way, the book is a Theodicy, an attempt to talk about God in terms of human suffering.
Marina forgives her father for causing her to suffer because the warmth and company he provides to her is irresistible. Her ability to forgive is the measure of her integrity, but also it's an indication that she has refused to play the victim role any longer. This is also a decent response to the theological problem of human suffering, and a psychologist would likely note that any issue with parental figures is likely going to effect every part of a child's psychology, so to a child, a parent is like God in a way, because they depend on adults for their provision, so when Marina's father accidentally abuses her, that is an interesting depiction of Marina's relationship to reality itself. She knows something about life that her classmates do not. She knows that humans suffer unfairly on the earth, so when she comes to peace, she comes to a deeper, more holistic peace than she might have otherwise, because she's coming to terms with her own suffering in a philosophical way.
The second most important consideration would be the way that writing allows Marina a voice, both literally, since she is rendered mute by her traumatic experiences, and figuratively, because the sweet, slow process of writing down intimate thoughts is extremely powerful and therapeutic. She essentially guides herself through her own healing process by just talking things through in her limited, sometimes frail, way.
