When President Lyndon B. Johnson and War on Poverty chief R. Sargent Shriver launched Head Start with great fanfare in 1965, there was confidence the preschool program would start impoverished children on the road to success. Now, with President Clinton poised to expand the program to serve every poor child in the nation, experts question whether Head Start children actually experience lasting benefits. Serious questions have been raised about virtually every benefit claimed for Head Start -- educational, social and health-related. Even Head Start's defenders are disturbed by the poor quality of staff and facilities at many centers. The prospect of more than doubling federal spending on the program has intensified the debate over boosting enrollment versus improving quality for a limited number of children.