Great title for kids to read when learning about WWII - it helps put them into that time period and experience it a little bit.
Another Ggreat title for this time period is 'Twenty and Ten' (also titled "The Secret Cave').
I read this book in my childhood, and was delighted to find that not only was it still available, it is still in print. The tale is a fun, adventurous read for children, but reading it as an adult, I realized what great risks the children were taking. Debate rages on as to whether these events ever took place, but it is a good story to introduce or remind children of other countries' involvement in WW II without being too scary. Recommended.