Don’t feel too bad, it was an honest mistake. Is your nephew reading at a high level? If he is you can have a discussion with him about the contents of the book and what he understands, unless his mother has completely nixed the book. I don’t really see an issue with it, but at 7 years old I was reading Pet Sematary, Firestarter, and The Shining and the like with my parents’ permission because they knew my comprehension level.