Children of this age group have a very distinct advantage over the other age groups.  They are capable of learning new patterns of movement.  Having little or no exposure to improper form and function gives a trainer a clean slate to start from.  No bad habits to erase!  This is where the 'programming' of muscle movement and muscle memory are etched into the Neuro-systems.  Learning proper movement patterns at an early stage creates a long lasting platform to build from.