Most cycle helmets consist of the same basic features. An outer plastic shell with an inner padded layer which spreads the impact, padded, air vents and an adjustable fitting system.
It is important that you buy a helmet that meets European safety standards (look for a “CE” sticket inside.
It is really important that a helmet fits the rider head comfortably and doesn’t wobbly around. Helmets can normally be adjusted so they fit a range of head sizes.
To find out what size helmet your child needs, take a tape measure and measure the circumference of their head about two finger widths above your eyebrows.