In her book The Mission Minded Child: Raising a New Generation to Fulfill God's Purpose (Colorado Springs: Authentic Publishing, 2007, pp. 2-4), Ann Dunagan lists some of the distinctives of kids who are developing a heart for missions.

A mission minded child...

• ...dreams of fulfilling God's destiny.
• ...May want to become a missionary--or a teacher or a doctor or a newspaper reporter or a state governor or a pastor or a businessperson or an airplane pilot or an author or a florist or a mother--as long as its what God wants.
• ...prays for that next-door neighbor.
• ...is not a picky eater!
• ...takes home a photo magnet from the visiting missionary family and puts it on the kitchen refrigerator.
• ...is healthy, active, and adventurous.
• ...spends a summer night sleeping outside on the trampoline, gazes up at a sky filled with twinkling stars, and realizes God's plan is infinitely bigger than his or her own backyard.
• ...imagines rollerblading on the Great Wall of China!
• ...recognizes the names of David Livingstone, Amy Carmichael, Hudson Taylor, and Loren Cunningham.
• ...knows how to use chopsticks.
• ...has a reputation for thoroughly enjoying the Bible sword drills and memory verse contests at church.
• ...puts extra money in the monthly missions offering and feels extra good inside.
• ...thinks it could be fun to sleep in a mud hut in Africa!
• ...reads all the way through the Bible by the age of ten (or eleven or twelve)--and is excited to start again!
• ...stares at the photos in the new geography textbook or magazine and imagines climbing to the top of that Egyptian pyramid, snorkeling in those tropical-blue waters, and giving a new outfit to that poor boy with the ripped-up shirt.
• ...befriends the new kid at school.
• ...thinks beyond the "box" of what's merely expected and hopes to do something big, or something little, for God.
• ...wants to obey (even when no one is looking).
• ...loves Jesus!