Author Russell Freedman mentions a number of WWI books for children in the bibliography of his excellent new book for young adults, The War to End All Wars. These include the following:
The Harlem Hellfighters: When Pride Met Courage, by Walter Dean Myers and Bill Miles (Amistad Press/HarperCollins, 2005)
Eyewitness World War I, by Simon Adams (DK Publishing, 2007)
Causes and Consequences of World War I, by Stewart Ross (Raintree Steck-Vaughn, 1998)
Lord of the Nutcracker Men, by Iain Lawrence (Delacorte, 2001)
Private Peaceful, by Michael Mopurgo (Scholastic, 2004)
War and the Pity of War, edited by Neil Philip (Clarion Books, 1998)
The Willmette (Illinois) Public Library offers a good reading list of WWI books for kids, also.
"Terrible Trenches," a WWI exhibit at London's Imperial War Museum continues through October 31st. Terrible Trenches is also the title of an upcoming book in the UK's "Horrible Histories" series for children. ("It's history with the nasty bits left in!") The prolific Terry Deary is the author. (See this interview with Deary, at the Guardian.)
You'll find additional posts on nonfiction books for children at the blog Write About Now, the host of Nonfiction Monday today.