Hermione's first instinct was to shield her son from the full truth. Yes, he knew bits and pieces from their previous conversation when she explained that Rodolphus couldn't be trusted because of the spells he used on her mind, but she didn't tell him everything. Part of her began to understand why Antonin was so determined to shield the ugliness of the truth from her. She didn't want to frighten Oliver or have him concerned about her well-being. Wasn't he allowed to just be a child? His childhood would be over so soon. It was tempting to carry the burden entirely on her shoulders, but considering how infuriated with Antonin she got for doing the same to her, she just couldn't bring herself to keep it hidden.