Crusade in Jeans, by Thea Beckman (1973 in Holland, 1975 in the US) is a classic example of the use of time travel to play out a thought experiment--how would a modern boy react to all the ramifications (physical, spiritual, and cultural) of the Children's Crusade?
Rudolf Hefting just wanted to travel back in time for a few hours. The time machine his dad's friends had invented had only been tested on animals, but it seemed to work just fine... And so the Dutch teenager is sent back to the Middle Ages, wearing jeans and sneakers, and armed with a bread knife, ready for three hours of observation, before the time travel field pulls him back.
But things go wrong. By unhappy chance, the time travel field kicks in just as the Children's Crusade is passing by--and instead of Rudolf returning home, one of the medieval children is taken in his place, leaving Rudolf stranded, caught in a seemingly endless procession of sick, exhausted children....
The dreadful plight of the children gives Rudolf a purpose back in the past. He travels with them from Germany to the tip of Italy, using his 20th century knowledge to serve as a de facto guardian angel, trying to save as many children as he can from a wild animals, the scarlet fever, starvation, hostile towns and farmers, a robber baron who demands children as tribute, a perilous crossing the Alps, and malaria....
Finally, when the Mediterranean has been reached, Rudolf faces his greatest challenge yet. The oceans did not part for the children, opening their way to Jerusalem, as the charismatic shepherd boy, Nicholas, has promised. Instead, it was all a plot to gather the children together and sell them as slaves.
Dolf is forced to be a hero, but he in his own mind is simply doing what is obvious--trying to keep as many kids as possible alive, and, in the process, to give some point to his enforced stay in the Middle Ages. As the journey progresses, his emotional involvement with particular kids grows (although not quite as much as I would have liked), giving depth to the story, and providing a nice counterpoint to the unrelenting series of perils they face. Once the rather cheesy time machine part is out of the way, it becomes an interesting story of courage and survival in time and place worlds away from modern Europe.
When it was first published in Holland back in the 1970s, Crusade in Jeans received The Golden Pen award (the most prestigious Dutch national award for children's books) Apparently, judging from the comments on Amazon, it lost something in translation, and indeed, although I thought the book was just fine, I wasn't blown away. Apparently the Dutch version is much more emotionally gripping, and the language more relaxed and colloquial--in Dutch, it seems to be a book to love with a passion.
That being said, the young English reader who loves all things medieval should find this fascinating.
Crusade in Jeans was a wildly popular book in Holland. It is being republished by Lemniscaat (November, 2011) in paperback in the US, and, mercifully, given a new cover (the one I used above). The older edition I read is shown at left, and it is a stunning example of how to make the early Middle Ages look really, really boring and dorky. Edited to add: I just spoke with someone from Lemniscaat at Book Expo America yesterday, who said the translation had been tweaked with a bit, which might alleviate some of the disconnect between the Dutch and the English response.
It was made into an English language movie (shown at right) in 2008. Apparently the Dutch won again--that version, which came out in 2006, seems to have been more complete than the US edition.
Viz age--When I wrote this review, Amazon gave 4-8 year old reading level recommendation, which is obviously a mistake. It's correctly up as Young Adult now, although it's perfectly suitable for upper middle grade kids (ie 10-12 year olds). Although Dolf is 15, there's no sex or language here that would automatically kick this into YA. Due to the disturbing situations in which the Children's Crusade finds itself, however (ie, lots of kids die), younger kids who are bothered by such things might want to avoid it.