Is it possible this parent is a poor conversationalist, and thinks this is a good topic because she thinks it is what you/your kids have in common? Or maybe she is just really looking for an opening to brag about her own daughter's accomplishments/acceleration? Either way... I guess I would not have had any qualms about saying that your son is very interested at the moment in Mesoamerican civiliations, and you are just trying to keep feeding him appropriate textbooks and information to satisfy him in that area. Then turn it around and ask what her little girl is working on these days. And turn the conversation to the unusual ingredient in the bean dip if you have to. I never really felt like I had to keep my D's interests secret. She was into entomology, Lord of the Rings, and wildlife biology when none of her peers were... but if someone asked, I would say those things.
If someone asked my kid's test results or reading level, I would say, "Oh, we really don't share that info." Then ask them a question about their own kid.
