The Children of the Lost: Book One in the Lost Mystery Trilogy by Franklin W. Dixon
The boys go on a chilling mission-camping in the woods where children have suddenly begun to disappear. Every time a child disappears, the word "LOST" has appeared outside their tent...and they are never heard from again. The boys think they've got things covered; they're ATAC agents after all. But when an "L" appears outside their tent their first night in the woods, things take a turn for the creepy.