I received the following question from a parent:
"When I pull the front brake, it works powerfully. But when I pull the rear brake it is very different and doesn't make the bike stop as hard and the bike skids easily. Is this normal for the rear brake to do this?"
Yes, this is normal.
The short version is that when the rider brakes, weight is transferred away from the rear of the bike and towards the front of the bike, which increases the friction between the front wheel and the ground and decreases the friction between the rear wheel and the ground. So the front brake becomes more effective and the rear brake becomes less effective. The exact ratio of effectiveness depends on a variety of factors including the weight of the rider, position of the rider, ground surface, tire material and tread, and wheel base of the bike. But a common range you'll see cycling enthusiasts throw around is that the front brake provides 75% to 90% of the total braking power.
If you want to do a little experiment with your child, show them how the front brake makes the bike stop quickly and the rear brake tends to skid. Then roll the bike backwards and repeat the experiment. When you roll the bike backwards, the rear brake should stop the bike quickly and the front brake will cause the front wheel to skid. This is because the momentum is reversed, so when you brake you are shifting weight to the rear wheel.
Learning how to use both brakes effectively is an important cycling skill. And it takes time and practice to master. I've seen many 4 year olds that can stop on a dime and many 40 year olds that still skid all over the place. Giving children the opportunity to start practicing this early is a big benefit of kids bikes with dual hand brakes.
It's a popular practice to encourage children to start by only using the rear brake. This method is popular precisely because the rear brake provides predictably poor braking performance. It's difficult (perhaps impossible) for a child to over do it with the rear brake. If a child slams on the rear brake as hard as they can, usually worst thing that will happen is that they will skid - which on a dry level surface, at little kid speeds, is usually pretty harmless (other than increased stopping distance and worn down tires). Conversely, if a child grabs too much front brake, they could stop so fast that they will get thrown forward on the bike.
Perhaps the worst side effect of encouraging children to use only their rear brake is that it tends to teach children to think of the right brake lever as the primary stopping control. This conditioning can be difficult to shake. I've met many adults that still think of the rear brake as the primary brake. As kids grow into mature riders, get faster, and venture out, it's likely that they will encounter situations that require them to stop quickly. A reliance on the rear brake lever will not serve them well in those situations.
Because of this, I counsel parents against teaching their children to be afraid of the front brake. I've seen parents tell their children that if they use the front brake they will get tossed over the handlebars and get hurt. I think this is a bad lesson to teach kids. First, I believe that this risk is exaggerated in the minds of most parents. I don't have any hard data about the risk of grabbing too much front brake, but my own personal experience based on years of riding bikes and being around other people on bikes tells me that few serious injuries are caused by over application of the front brake. Second, whatever risk exists is somewhat mitigated by the weak braking force small kids are capable of applying (Pro-tip: You can also loosen tension on the front brake cable to make the front brake even less effective). Third, and most important, the front brake will ultimately be an incredibly important control - arguably the most important control for crash avoidance. If they are taught to be afraid of the front brake, at some point this fear will need to be unlearned. The caveat here is that if a child is observed quickly grabbing the brakes (particularly the front brake), it's important to coach the child to apply brakes smoothly.
I see lots of kids (and adults) that are pros at modulating both brakes, but skid their rear wheel every chance they get. Kids like to skid their rear tire because it's fun. If you see your child skidding, it's important to understand whether they are skidding because they don't know how to brake more effectively, or if they are just having fun. Both of my children know how to use both brakes. And they both have fun skidding their back tires. They leave some skid marks on our driveway and go through rear tires quickly, but I'm OK with it because I did the same thing as a kid and I like that they are having fun on their bike. As a parent you can make your own decision about how to handle a child who has fun burning through back tires and leaving marks on the driveway - but most bike professionals that I know think a kid who is having fun skidding their rear tire is a pretty good thing.
All riders eventually should learn to modulate both brakes to stop effectively. In fact, some experts will tell you that on dry clean asphalt the fastest way to stop a bike is with the front brake only. This is because the fastest way to stop a bike is to apply the front brake hard enough that virtually all of the weight of the bike and rider is shifted off of the rear wheel and onto the front wheel. In this situation, because there is no weight on the rear wheel, it has no traction to provide braking power. While this theory is sound, braking hard enough to just barely lift the rear wheel while maintaining solid control of the bike requires a fair degree of skill. So this isn't a technique I would teach children.
What's important is that as parents we understand our child's braking skill and keep them in a place where their current skill level is enough to keep them safe. This last guideline is not just for brakes - whenever I introduce a child to any cycling situation that has any kind of hazard, be it traffic, a steep hill, or challenging terrain - I make sure that the child is ready for that situation.