Look for the roundest and smoothest helmet you can find so that it will skid across pavement easily during a crash. Check for an ASTM, Snell, or CPSC sticker inside. Put the helmet on your child and make sure that you cannot work it off when it is fastened and the straps are adjusted. Look for bright colors on the outside for visibility. Heads grow less than feet, so helmets do not need to be replaced often.
Some helmets come with several sets of foam fitting pads.
You can start off with the thicker ones and use thinner ones as your child grows. The fitting pads do not affect the safety of the helmet, which is provided by the firmer crushable polystyrene foam. Replace the buckle if it cracks or any piece breaks off. Replace any helmet after your child crashes in it because the impact crushes some of the foam. Even though the damage may not be obvious, the helmet may no longer be protective.