A child's bicycle will need basic maintenance to keep it running smoothly. If you find the brakes aren't working correctly, then you can fix them yourself. There's no need to go to a professional bike shop to fix the brake tension. The whole tension correcting procedure should take less than 5 minutes and can be completed using a common tool.
Find the hand brake, which is attached to the handlebars. Grip it with your hand to activate the brake to determine if the brake needs more tension or less tension. A brake that's too tense will be hard for a kid to squeeze, while a loose brake won't effectively stop the bike.
Look for spot where the cable goes into the hand brake. Notice there are two moving parts. One is a nut that is touching the brake grip, and the other is a knob connected to the nut. No tools are necessary to modify the tension of a kids' bike.
Turn the nut counterclockwise until it's loose.
Twist the knob counterclockwise to loosen the brake tension, and clockwise to tighten it. Continue squeezing the brake in and out while you're adjusting the knob and stop when the tension feels right.
Tighten the nut by turning it clockwise.
Turn the bicycle upside down so it's resting with the seat and handlebars touching the ground.
Spin the tire and see if the brake pads are rubbing against the rims of the tire.
Take a wrench and loosen the piece of metal bolt holding the brake pad in place by turning it counterclockwise.
Adjust the brake so that it's square and barely touching the rim.
Tighten the bolt using the wrench by turning it clockwise.
Be careful not to get your fingers stuck in the spokes when you spin the wheel.