THE best Christmas gift many American children could receive is one they shouldn't have to put on any list: a good start in life.That sounds self-evident. Yet more than one-third of the nation's children start school educationally, emotionally, and socially unprepared, according to the Carnegie Foundation for the Advancement of Teaching. Nearly 2 million kindergartners lack the language skills and emotional maturity for learning, the foundation says. The group's report, "Ready to Learn: A Mandate for the Nation," calls for a decade-long national campaign to improve children's readiness for school. Its goals include giving all children a healthy physical start and a quality preschool experience. Parents must be trained to serve as children's first teachers, the report states, and employers should support families by providing parental leave and flexible scheduling. The foundation also calls on television networks to offer at least an hour of quality preschool programming a week, and encourages communities to provide space where children can engage in safe, creative play. These are laudable goals. But it will take more than a report to translate ideals into action. Without firm, broad-based commitment from politicians, teachers, employers, and parents, the Carnegie report, like many other well-intentioned efforts before it, will simply disappear into filing cabinets. Child advocacy groups are intent on putting children front and center in 1992 political campaigns. More than 40 such groups have formed the Coalition for America's Children to push for a stronger government role in improving the health, education, security, and safety of all children. But even the best government efforts, of course, can never take the place of strong family support for children. One kindergarten teacher in Minneapolis observes in the Carnegie report that young children "need more lap-time with their parents so they know they are loved." Calling for an investment of time as well as money gets to the core of the report, with its heartfelt conclusion: "Preparing all children for school requires, above all, imagination and will - not vast amounts of money."