Generally, when an author writes a book or piece of literature, they title it something significant that ties into the story. This is certainly the case with A Child Called "It". Essentially, the novel details the life and times of Pelzer. If you think back to the story, it begins with Pelzer describing his final time coming to school, and lying about what happens at his home. From the very beginning, readers get the sense that Pelzer is somehow detached from his reality. He almost details his life from the perspective of a third party, observer, though he is telling the story from his point of view. This is one clue as to why he chose to call the novel, A Child Called "It". After all, the novel is an autobiographical account, but Pelzer's reality is so troubling that he deliterately separates from it.
The second clue as to why he titled the book this comes when he describes the deterioration of his and his mother's relationship. She went from watching the sunset with him, to beating, starving and referring to him as the boy or it. Thus, he titled the book A Child Called "It" to foreshadow the triumph that he overcame with his mom. Just as he left her, he also left that life.