Here is an excerpt from an interesting piece on federal Preschool Development Grants (and preschool programs generally) by a fellow at the Brookings Institution,

State pre-K, like Head Start, is a program with many staunch advocates and no reliable data demonstrating long-term positive effects. And both pre-K and Head Start are proposed for increases in funding in next year’s federal budget. The danger is that the opportunity to help poor children is being squandered on poorly conceived programs that do not accomplish what is hoped for them. In fact, both programs are inaccurately described and understood in their reality, but exist instead in some idealized fashion in the minds of policy makers and advocates. No new policy initiative should be launched without an accompanying rigorous evaluation of its effects. Researchers should insist that policies like the Preschool Development Grant are evaluated objectively to determine if they actually address the problems for which they were adopted to solve, and if they do not, are reworked until they do. It is a disservice to children to do otherwise.

Indeed, good intentions are not enough.  Quality programs for low-income children and rigorous evaluation of those programs are essential.