This is Dr. Steven Andrew Davis, Speaking of Health. Closed system sports are ones where activity conditions are fairly constant and there are not a whole lot of decisions to make. In a swimming competition the object is to cross the pool as fast as possible.
Young children tend to adapt better to closed system sports than to open ones.
In another closed system sport, like gymnastics, the focus can be undiluted by a multitude of other factors. In an open system sport, however, like baseball or basketball, the variables are many. In an an open system sport the burden of mental decisions takes away from the child's need to concentrate on fundamental skills like running or jumping. To apply these principles to your kids or grand kids you don't need to be a pediatric exercise physiologist. Just watch the child participate in several sports and, if it becomes clear that the variables are too overwhelming, move him or her into a closed system where the goals are straight - forward and attainable.