If I may make an analogy, Terry's books are like the ocean: on the surface, they are light - "only for kids" to "splash around in" and have fun. But beneath the surface, there are great depths and problems to fathom and think over.
On a more light-hearted note, I wonder what passed between them. Did the Queen say "So you're a famous author, then? That's wonderful. And what does that involve?"