First, I ask if anyone knows what the instrument is (guitar is a usual answer), then I play How Would You Like A Great Big Ice Cream so they can participate by making movements and coming up with new things to like such as cookie, pizza and other things to overcome my ideas of onion and broccoli.
Then I pick If You're Happy and You Know It while talking about how nice the guitar sounds. Every class I've ever done, even 3-year-olds, recognizes the song so they get excited and can hardly wait to sing. Ask if they sing, "if you really want to show it" or if they sing, "and your face will surely show it."
After the complete song (I usually play two verses, then move to mandolin and play two more for preschoolers), then I discuss how we made noises from all three instrument families. There was my Guitar for strings, hand-clapping and foot-stomping or knee-patting for percussion, and what did we do for Wind? I reinforce the soundbox concepts in older classes by saying my voicebax is bigger so I can make lower notes and my lungs are larger so I can make louder notes.
Put on the fingerpicks and show how they fit your fingers and then demo the twang sound.
I play three verses of She'll Be Coming Round the Mountain When She Comes then say that's the end of the song (Usually I have a poster or papers with all the extra movements and sounds the kids make so they know I'm teasing).