The Codebreakers is a fantastic book and I highly recommend it. I don't know what I think of it for a ten-year-old, it's been a while since I've read it, but my memory is that it is certainly mathematically accessible, but its length and depth may make it daunting, as it probably exceeds the attention horizon of a typical ten-year-old by an order of magnitude or two. But in chunks it could work, and for an adult it's fantastic.

It does assume a fair bit of historical context knowledge, though, much of which probably exceeds 5th grade. Probably 9th graders would have it easier as far as, e.g., familiarity with WWII, etc.