The Secret Project Notebook by Carolyn Reeder

Fritz (a.k.a. Franklin) is determined to find out what makes the small southwestern town where he now lives so secret that it's not on any map and no one is allowed to say or write its name. He and his seventh-grade classmate Kathy join forces as they attempt to decode phrases like "tickling the dragon's tail" and to discover where their scientist fathers go when they disappear for days at a time. But it's a summer prank rather than their snooping that causes the military police to view Fritz and his friends as a threat to wartime security.