Young children should use bikes with coaster brakes. These are the kind that brake when pedaled backward. With hand brakes, a child's hands should be large enough and strong enough to squeeze the levers.
Make sure bikes are the right size, rather than a size "your child can grow into." Your child should be able to straddle a bike with both feet on the ground. Children cannot handle oversize bikes and are at risk of falling and other accidents.
Even when riding on sidewalks, children need to learn to watch for cars pulling out from driveways and alleys. Also, teach children to watch for wet leaves, gravel, and curves.
Make sure your child is careful about keeping loose pants legs, straps, or shoelaces from getting caught in the spokes of the wheel or bicycle chain. Teach your child to never ride barefoot, or while wearing sandals or flip-flops.