There are three main types of neoprene headgear: beanie, cap or balaclava. Most surfers will use a balaclava style as these cover the neck and tuck into your wetsuit. A cap has a chin strap rather than coming down around your neck, most have a peak but some swim specific styles come without the peak. 
A small adult size fits most children but for younger kids we have a one size fits all adjustable balaclava.