For the Terry Pratchett fans out there, nothing more need be said. It's Pratchett, you want to read it, the only reason you've been hesitating is because it's marked as a kids book (juvenile, young adult...) But this one isn't just for kids. As with any Pratchett book, there are layers and layers, and some of them wouldn't be obvious to kids at all.
For example, kids who have only seen the Batman movies, and not the original TV show, will miss it entirely when Mrs. Tachyon is saying "dinner, dinner, dinner, dinner..." and continues a few more times between interruptions, finally ending with "dinner, dinner, Batman!" which is where adults (at least my generation) will realize she's not saying dinner, she's humming the theme song. Also, kids the age of our protagonists, 13 or so, may not recognize the "red shift" when they get to it; that's usually covered a bit later in the science curriculum, such as college physics.
The protagonists are Johnny, and his friends Wobbler (who wobbles), Bigmac (who is large), and Yo-less, who is apparently the only black in Blackbury who doesn't say yo. They are joined in this book by Kirsty/Kasandra (she changes her name each week), who is hyper-intelligent and socially even more inept than the others. Each of this team has his own strange store of skills or knowledge. These talents turn out to have entirely different implications when travelling in time than they do in their own time. Bigmac's car-stealing abilities (which some parents may object to in a kids' book) turn out to be impaired when trying to steal a car that doesn't have power steering and power brakes. On the other hand, Yo-less's lack of cool is suddenly changed when he puts on period clothing and suddenly looks, as Johnny says, as though he plays the saxophone in a band. Yo-less does, though get exposed to the more primitive social prejudices of 1941, as does Kasandra. And Bigmac finds out that the skinhead symbols and attitudes that he wears only as a social item suddenly have real meaning, and it's not pleasant. OK, there's a bit of a moral or two snuck in here, about thinking about what things mean. There is also at least one moral that readers one and all will ignore, just as the characters do, about following advice (and about giving it).
Johnny has been working on his World War II project for school since the previous book, "Johnny and the Dead." One of the funny bits in the book is how, whenever a kid claims he's doing "a project," he winds up with all sorts of information that is unsuitable for kids, and/or hitherto classified or secret; the remembered horror of school projects makes all the adults give in so that they don't have to think about it any more!
Other reviewers have described much of the plot, so I won't repeat it here. One thing that some readers may wish to note about this plot is that it isn't just time travel, it's alternate history as well, and for kids this may serve as an introduction to the whole sub-genre of alternate history. Meanwhile, some of the high points:
* Mrs. Tachyon's cat, Guilty - and his tastes in food.
* The ice that forms on the characters during their last-minute rush for the air-raid siren.
* The importance of pickles.
The series has no noticeable sexual content, and no real bad language; the most dangerous things in it for young readers are the ideas, which may make them *gasp* think! It may also make them lifelong Pratchett addicts. In the opinion of an existing Pratchett addict, there's nothing at all wrong with that!