This is horrible reasoning. Thinking back to books I read as a child, in plenty of cases I was just oblivious to when they were set; it just wasn’t relevant. (As a child in the 90s I probably wouldn’t have noticed if a book was written/set in the 70s or 80s and maybe even earlier.)

And then in cases where the time period was obvious… so what? Children aren’t stupid. I read Famous Five books and I was also aware that my mum had read them as a child too.