i've had two successes in teaching gears to kids. one is off the bike, one is on. both, unfortunately, require a short, easy hill students can walk/run/ride up and down safely. on this hill, mark out a "course". this is just a simple start line and finish line. could just be 4 cones or halved tennis balls on the left and right of the course.
1. off bike:
without getting into the math of gear ratios, etc, talk about stride. ie, when you're walking or running up a hill, you take shorter steps. when you're walking/running on flat ground or downhill, you take longer steps.
have students walk UP the hill taking many many short steps, and count the steps it takes them to go from "start" to "finish. then have the students do the same while taking half as many steps. have students think about which one was EASIER on their legs. (not necessarily faster.)
2. on bike:
have students line up, on their bikes, at the start line. make sure everyone is in the highest gear. also be clear that it's not a race. have them ride up the hill. once everyone's done or has given up, have them shift into their lowest gear and repeat.
talk about how #2 above is related to #1. in a lower gear, it's easier to pedal, but you don't go as far. if you have time or interest from the students, or students who get the above but need more of a challenge, have them calculate the approximate distance their bike goes in one revolution of the pedals in low gear, and the same in high gear. sometimes this helps drive the point home as well.
good luck!