Terry Pratchett is, of course, best known for his Discworld books, but he also wrote (among other things) a three book sci fi/fantasy series for readers 9-12, about a boy named Johnny Maxwell and his friends. Johnny and the Bomb, the third book (1996), takes Johnny and co. back in time to World War II, just as their town is about to be hit by German bombs....
Johnny knows the bombs are coming, and that people will be killed because the air raid siren isn't going to off and warn them. If he can sound the alarm, he can save them...but caught in the temporal paradoxes of changing the past, and hampered more than he's helped by his companions in adventure, he might not be able to.
Johnny and his friends are a somewhat confusing bunch of mis-fits (three boys, and one girl)--they are all rather mad, in the British sense of the word. The madness that they create just by existing is compounded when they encounter the shopping cart of a bag lady, who just happens (though they don't know it) to keep time (or something very like it) in the grotty plastic bags she wheels around. When Johnny and the friend who is a girl (mostly named Kirsty though sometimes she chooses not to be) start poking at the cart (not that they really wanted to, but these things happen), it starts whisking them through time.
And eventually all five kids are back in 1941, not adding much to moral, and not, at first, realizing that if they don't do something, the bombs will kill the very people they are meeting. It does not help that one friend has decided to travel through time wearing a German uniform.
I rather think that I had read the other two books first, I would have been altogether calmer and more receptive, happy to see Johnny and all instead of confused and unconvinced by them (although not un-entertained). But I had not, and so I was. Fortunately, I was curious enough to continue on (chuckling, it must be said, quite often), and was rewarded by a cracker-jack time-travel paradox gem when Johnny must slide around the linear path of time to sound the alarm. That part was really good (or fully realized, if you want something fancier).
Short answer: read the first book first. Read this one first only if you are a. a passionate devotee of WW II juvenile fiction b. reading every time travel book for kids you can.
Bonus: interesting bit of grim humor regarding how the residents of WW II England might react to a black boy (one of Johnny's friends)
Final note: it is never explained how or why the mysterious bag lady and her shopping cart travel through time, so don't expect to be any wiser by the end of the book.