It was a good title for the original book. I don’t know if it’s just a British thing but there is a tendency here to acronymise things we are anxious about talking about, it must be universal because I have heard Americans refer to Cancer as “the big C” as if mentioning it by name will bring it upon yourself. As the title is identified closely with the book, keeping it for the film makes sense. Parents and kids familiar with the story will recognise it as an adaptation. They might not do otherwise.