-Since spy/detective work relies on the power of observation, put out 12-24 random objects and let the kids observe them for 30-60 seconds, then hide the objects and see how many they can recall. You could also make/use invisible ink, periscopes (to see around corners), fingerprinting (baby powder makes these show up), and other "detective science."
-How about a scavenger like game? You could hide the clues around the library with easy questions about the library or about storybook characters.
-the FBI and NSA has some cool stuff on their sites: http://www.nsa.gov/kids/ http://www.fbi.gov/kids/k5th/kidsk5th.htm
-This might do; it is called "Poison Candy". Put a handful of candy on the table and have one person leave the room. The group decides which piece of candy is poison. When the person comes back in they choose one piece of candy at a time and they get to keep each piece until they pick a "poison" one. I play this with 4-6 graders and they get really thoughtful about the whole thing. If the child has a retainer then they won't pick gum because the person can't chew gum so they wouldn't choose gum, or they may know a certain person loves chocolate, etc...
-Wink Murder works well. It's also called Mafia or Murder in the dark. You need a deck of cards. Take all the face cards out except for 1 king and 1 joker. Deal out the cards to the kids, and then have them mill around in a group, saying "How do you do?" or whatever to each other. Whoever gets the joker is the assassin and walks around winking at people. Whoever he winks at has to count to five and then die as dramatic a death as possible. Whoever gets the king is the detective and has to figure out who the assassin is. At the beginning of the game, whoever the detective is has to own up to being the detective; everyone else has to keep their identity a secret. Call the assassin and the detective spy and spy catcher if that makes it fit the theme better. It's a flexible game.
-How about a 'spy' version of Kim's game, to test powers of observation. Or, conduct a scavenger hunt. Divide the group into pairs or teams, give them envelopes containing their 'mission', maybe written in 'invisible' ink or mirror written. Send them on a hunt throughout your department, or if the rest of the staff is open, throughout the library. Design the clues so they have to use their library skills. Have their reward waiting for them when they complete their mission--food!
Spy/Adventurer Relay Race. The relay race where everyone has to put on clothes, in this case spy or adventurer clothes (baggy khakis, button down shirt, shoes, trench coats and hats. Check out games on the internet and in books and add adventure or spy element to it.