Yeah, there are some pretty "adult" things going on in this book. We don't mean violence, sex, or anything like that, but there are elements that usually aren't considered kid-friendly topics. Art history, for example, or the ravages of aging (as seen through Mrs. Frankweiler's face). Nonetheless, this is a story about kids, for kids.
It's a good old-fashioned young adult mystery, where the two young protagonists work together and discover a secret that adults weren't able to. In the same vein as young sleuths like the Hardy Boys or Sammy Keyes, Claudia and Jamie never give up. And they find some pretty good clues despite their inexperience.