Now that I've finished Beasts of No Nation, I have many questions, as do other Readers, I'm sure, about child soldiers and am ready to turn to nonfiction for some answers. Here are some suggestions,
First, Amnesty International has created a curriculum guide to accompany the book Innocents Lost: When Child Soldiers go to War by Jimmie Briggs.
Another nonfiction book on the subject is P.W. Singer's Children at War. You can get a sense of the book from this video presentation (30 minute talk plus 30 minutes of Q&A). I found this very informative.