Cantilever (Center Pull) Brakes
Cantilever brakes are thankfully fading from existence, being replaced by the much better V brakes.
Cantilever brakes are center pull brakes--a bridge cable connects the 2 brake arms, and the cable from the brake connects to the center of the bridge cable. V brakes are superior because they deliver much more force to the brake pads, and because their design is simpler and therefore easier to work on.
Cantilever brakes were found on almost all mountain bikes up to the mid 1990s. Now some cheap department store bikes come with cantilever brakes. Some new cyclo-cross race bikes use cantilever brakes as a matter of style, even though V brakes would work better.
The best thing to do with cantilever brakes is throw them away and replace them with V brakes. V brake sets are available from $50 to $100. You'll need front and rear brake arms and V brake levers--cantilever brake levers won't work on V brakes. If you bought your kid a cheap department store bike with cantilever brakes, shame on you. Repent and buy a bike that will actually stop from a professional bike shop.