The theme of A Child Called "It" is the various forms of child abuse. This autobiographical book simply records the plain facts about the truly horrible physical, verbal, and emotional abuse inflicted on Dave Pelzer by his own mother. The woman was quite obviously mentally ill, but she managed to continue abusing one of her sons, the author of the book, for years without being exposed. The book is so harrowing that many people cannot bring themselves to read it all the way through. It is almost like something out of the Marquis de Sade. She called her son "It" because she hated him, treated him like an animal, and apparently wanted him to think of himself as such. She had three sons, but Dave Pelzer was the scapegoat who endured by far the worst treatment.
Psychologists have identified a condition which they have termed the Medea complex, after the Greek play by Euripides (431 BCE) in which Medea murders her two sons as an act of revenge against her husband Jason who has abandoned her. Here is an excerpt from the summary (or abstract) of the article on the Medea complex in the British Journal of Psychiatry, which is accessible on Google.
The situation in which the mother harbours death wishes to her offspring, usually as a revenge against the father, is described and named the Medea complex.
It is shown that there is considerable resistance against admitting these thoughts to the consciousness of the mother or any other person, but that they are of general occurrence.
Note that the author Edward S. Stern states that these death wishes "are of general occurrence." In other words, there are a great many children being abused by their mothers, but hopefully not as severely as was the case with David Pelzer.