Kids are often entranced by spies who sneak around and lie yet are still viewed as "good guys." For your child's party, consider using a spy theme. Kids will love acting like detectives: making question mark cupcakes, following clues to find a treasure and comparing each other's fingerprints.
Definitely stay away from typical party invitations to a spy party. You can either write the information as a mirror image (so that the recipient must hold the invitation up to a mirror to read it), write it in code or use lemon juice and instructions about how to read the secret message. Rather than placing it in a regular envelope, mail it in a small manila envelope with a "Top Secret" sticker over the opening.
A few well-placed decorations will make your spy party individualized. Cut out some footprints and place them leading up the path to the front door of the party location, create a "Most Wanted" poster with a silhouette on it to put on the front door, plaster "Caution: Crime Scene" tape along the walls and hang some pictures of the birthday girl or boy on the wall with an added mustache, beard, hat, glasses or other accessories added in black permanent marker.
You'll want to prepare several activities for your child's spy party to keep the kids busy and entertained. One of the most loved spy-related activities is a gumshoe treasure hunt in which kids follow clues that lead them from place to place until they finally find a "treasure" (e.g., party favors, the birthday cake or some other treat) at the end. You can also build an obstacle course outdoors, and tell kids that spies are physically fit to get out of sticky situations. Alternatively, ask several kids to dress up in spy clothes (long trenchcoats, boots, hats), stand backwards, and tell the rest of the guests to guess which kid is which.
Try some fingerprinting crafts with the kids, letting them dip their fingers into different colored ink pads and using their fingerprints to make interesting designs. You can also let them frost mystery cupcakes, decorating them with question marks made of frosting, miniature chocolate chips or sprinkles. For a bit of extra fun, prepare the cupcake batter before the party, and ask the kids to write one clue about themselves on a piece of paper, fold it up and drop it into one of the cupcakes. When the time comes to eat the cupcakes, kids can "eat out" the clues and try to figure out who wrote each of them.
Good party favors for a kid's spy party include magnifying glasses, ink pads for kids to make more fingerprints, sunglasses and fake disguises, such as glasses with a nose and a mustache attached.