Before choosing any kids' bike wheels, you will need to figure out which size is appropriate for the model of bike your child owns. Kids' bikes come in a variety of sizes to accommodate kids of varying skill levels and sizes; very often the only reason you will need to buy kids' bike wheels is if the wheels that came stock on the bike bend so severely that they cannot be trued, or straightened. Building a custom bike for a child is usually not recommended because it will end up costing more than buying an entire bike and the child can grow out of the bike quickly.
The size of the kids' bike wheels can be determined by looking at the bicycle tire. In most cases, the tire size will be written on the casing, and the format will look something like this: 16x1.95. The number 16 indicates the tire's diameter, while the number 1.95 indicates the width of the tire. The wheel size is determined by the tire diameter, so if a 16 inch (40 centimeter) tire is currently mounted on the child's bike, the replacement wheel will also be sized at 16 inches (40 centimeters).
The most common kids' bike wheels are sized at 12 inches (30.5 centimeters), 16 inches (40 centimeters), 20 inches (50.8 centimeters), 24 inches (61 centimeters), and 26 inches (66 centimeters). 14 inch (35.5 centimeter) tires are also available, but they are less common than other sizes. Do not be surprised if you see the size written in inches only, as many bike tires do not include a metric measurement. The specific wheel size is designed to work with a specific frame size; 12 inch kids' bike wheels, for example, will not work safely or properly with a frame that is designed to accept 20 inch wheels.
You may also consider purchasing training wheels, which extend off to either side of the bike to provide stability to a child still learning to ride on two wheels. These wheels are made of hard plastic or rubber attached to metal arms; be sure to buy training wheels that are stable and sturdy as well as adjustable, and be sure to buy the correct size training wheels. The size of the training wheels you will need will depend on the size of bike wheels the bike features; a 20 inch bike, for example, will require training wheels that can accommodate that wheel size.