The Children's Book by A. S. Byatt

Famous author Olive Wellwood writes a special private book, bound in different colours, for each of her children. In their rambling house near Romney Marsh they play in a story-book world - but their lives, and those of their rich cousins and their friends, the son and daughter of a curator at the new Victoria and Albert Museum, are already inscribed with mystery. Each family carries its own secrets.
They grow up in the golden summers of Edwardian times, but as the sons rebel against their parents and the girls dream of independent futures, they are unaware that in the darkness ahead they will be betrayed unintentionally by the adults who love them.