We're getting children on bicycles with derailleurs at a very young age. Because of that, we often get asked how to teach young children to shift. Here at Prevelo I've been successful at teaching kids as young as 3½ to shift. Every 4 year old that I've worked with has been able to shift using the Shimano Acera RAPIDFIRE shifters that come on the Alpha Three and Alpha Four. To be clear, I'm not implying that all 4 year olds should be able to shift (in fact, size wise few 4 years olds are ready for the Alpha Three, which is our smallest bike with a derailleur). But with proper instruction and quality shifters most young children can learn to shifter easily.
Hands down my preference for young children is a Shimano RAPIDFIRE style shifter. This type of shifter has two levers. One is lever operated with the right thumb and the other lever is operated with the right index finger. The shifting is binary and easy to learn. And the action is smooth and well indexed.
Here's a little handy chart to remember what each lever is for:
Moves to Larger Cog
Moves to Smaller Cog
Lowers Gear Ratio
Raises Gear Ratio
Going fast or downhill
Teaching: Step 1 - Learn the Levers
I like to start by introducing children to the two levers and how to use them. This entire step happens with the bike standing still in a safe and level place. The rider isn't moving or pedaling. I want the rider totally focused on those two black levers on the right side of the handlebar.
Put the bike into a mid-range gear
Have the rider sit on the bike and grip the handlebar with both hands.
Guide their thumb and forefinger to find the two levers. Teach them to identify the two levers. Point to the thumb lever and say "this is the thumb lever". Point to the finger lever and say "this is the finger lever".
Have the rider pull the finger lever twice. Then have the rider pull the thumb lever twice. Repeat this over and over until the rider can easily find and operate each lever. (It's important to start with the finger lever since this will slacken the shifter cable).
Goal: Before proceeding the rider should (1) be able to find and operate both levers and (2) respond to your instructions to "pull the finger lever" and "push the thumb lever" - or simply "finger" and "thumb."
The Shimano Acera shifters that come on the Alpha Three and Alpha Four have the ability to shift either one or two click per stroke on the thumb lever. The first click will move the chain 1 step into a larger cog. The second click will move the chain 2 steps into a larger cog. At this step in the learning process, I'm not concerned about whether the rider is shifting 1 or 2 clicks on the thumb lever per stroke, as long as they are consistently getting at least one click.
When the thumb lever is operated, it loads a spring in the derailleur. Because of this, it takes some force to operate. Children are creative at finding ways to use all of the force their little bodies can muster. Here's a 3.5 year old shifting an Alpha Three. He's too small for the bike but can still operate the shifters by engaging his full arm and even some of his upper torso muscles in the action. He's actually leaning his entire upper body into the shifter. If your rider is very small and struggles with the force required to downshift, encourage them to experiment with different ways to get leverage on the thumb lever. Remember this isn't an electronic button (unless you want to splurge for a Di2 upgrade). The rider is pulling a cable, which is in turn both moving a derailleur and chain, and loading a spring. Very small children (particularly those 4 and under) will need to be assertive about pushing the thumb lever. Be patient and give them time to experiment with how to harness their strength to push the lever with force.
Note that when you are using the shifters without spinning the pedals, the chain cannot actually move into the correct gear on the cassette. So the derailleur will become misaligned with the chain. This can put tension on drive train components. However, Shimano drive train components in the Prevelo line are very robust and difficult to damage with this practice, especially with kid sized force. But some good guidelines are (1) don't shift more than two cogs larger (two clicks of the thumb lever) from where you started and (2) if the thumb lever feels extremely hard to push, don't force it (instead shift with the finger lever first to create slack in the shift cable, and then try the thumb again).
Teaching: Step 2 - Learn the Mechanics
Once the rider is proficient at operating the shifters, I like to have them see what they actually do.
Have the rider stand on the right side (drive train side) of the bike.
You stand on the left side of the bike.
With your right hand grab the back of the saddle and lift the rear tire off the ground.
With your left hand spin the pedals forward.
Instruct the rider to "pull the finger lever" and have them watch how the chain moves on the cassette.
Instruct the rider to "push the thumb lever" and have them watch how the chain moves on the cassette.
Goal: The objective here is to help the rider understand that (1) when the shifter levers are operated, the chain moves on the cassette and (2) that this only happens when the pedals are spinning. The later is an important lesson - in order for shifting to happen the rider must both (1) operate the shift lever and (2) spin the pedals.
Be careful when spinning the pedals on a stationary bike next to your child. When the pedals are spinning the entire drive train and rear wheel are hazardous areas for fingers. Also, when lifting a bike by the saddle, make sure that both seat post clamp and saddle rail clamp are both tight and secure.
Teaching: Step 3 - Shifting Gears!
Now that the rider knows how to operate the shifters, it is time to bring it all together.
Shift the bike into a midrange gear and have the rider start pedaling. An adult should spin the pedals to make sure that if the bike was shifted while stationary, that the chain is settled into gear before the rider mounts the bike.
Instruct the rider to "pull the finger lever". Make sure the rider continues to pedal until the shift is complete.
Instruct the rider to "push the thumb lever". Make sure the rider continues to pedal until the shift is complete.
Continue this step until the rider can consistently shift both up and down on the cassette.
Getting More Advanced
The next step is for children to learn when to shift and what gear to shift into. As long as the rider responds to your "finger" and "thumb" instructions, you'll be able to coach the rider to shift into the gear that you believe is correct for the terrain. For example, if you approach a steep hill you might instruct the rider to "click the thumb lever 3 times". From my experience, through this type of coaching children quickly catch on to when they should be shifting and the gear that they should be in.