A look at how America's love affair with prescription drugs has moved into the playpen. Today, 4 million children begin their school day by taking a small yellow pill, Ritalin, to control their hyperactivity. Since 1990, there has been a 700% increase in the use of the stimulant. And more than 2. 5 million prescriptions for anti-depressants were written for preschoolers and adolescents. We look at all sides of the issue and talk to pediatricians, psychiatrists, teachers, parents, and children.