On two occasions I purchased reasonable quality used bikes for my kids that had cantilever brakes. As has already been mentioned, you need to have at least some mechanical aptitude to set them up right. There are several degrees of freedom in the pad clamping and cable adjustment that can take them from excellent to downright scary.

After some fussing with the most recent acquisition, a Trek Mountain Lion, I decided to convert the system to linear pulls. I grabbed a suitable donor bike on Town Junk Day, and stripped off the needed hardware.

In theory, you need to match up handlebar levers with the style of brake. The mechanical advantage or cable take-up rate is different, and it is generally not recommended that cantilever levers be used with linear pulls. The results have been labeled 'too powerful' or 'touchy'. I did some quick measurements and decided to proceed with adding the wheel components without changing the handlebar levers.

I let my older daughter try it out, and she was quite surprised by the results. Her comment: "wow, I didn't expect the brakes to be so powerful!!!" The result is a system that does not take a lot of squeeze to produce a lot of braking force. Is this a good thing? For you as a mountain biker, maybe not! You may find them too hard to modulate. For a little kid, this is a great breakthrough, as most children find it very difficult to get their hands around levers and squeeze hard enough to brake effectively. Go to a big box store and put your hands on the kind of bikes most parents buy their kids. I'm not sure how some young riders can do it, or how manufactures can put out these products in all good conscious.