IN AN urban Head Start classroom, a 3-year-old has just been dropped off by her father and sits on the lap of a volunteer from the "Granny Brigade." As he leaves, she begins to wail, "Daddy! Dadeeeeee!" Her "granny" holds her gently until she calms down and is ready to join other young children in their morning activities. They go through this ritual for several days until the little girl gradually adjusts to the new experience known as preschool.