Most kids' bikes (single speed) are equiped with pedal brakes that stop the bike when pressing backwards on the pedal. These are generally easier for children to use than hand brakes. If the bike only has one hand brake, it's also equipped with a pedal brake.
When choosing a bike with two hand brakes, ensure that your child can reach and pull the levers. Many are difficult for kids to use because they can be either too big for their small hands or too stiff. Almost all hand brakes can be adjusted and lubricated to suit kids' hands better.