There is a big difference between a one-off accident, such as beng hit with a cricket ball, which may never even happen, and deliberate regular contact which happens frequently during a game. Ths a particular concern with young children who have skulls which are not fully formed and more prone to damage.
Besides if children below a certain age were banned from heading they may learn better ball playing skills which in turn could lead to a more entertaining passing game rather than hoof ball to a lone target. Just a thought.