If you're still on the fence about whether quality early childhood education matters, consider this:By age 3, inequality in the United States is clear: Rich children attend school. Poor kids stay with a grandparent or other caretaker, according to research by authors of a new book, Cradle to Kindergarten: A New Plan to Combat Inequality.Only 55 percent of America's 3- and 4-year-olds attend formal preschool. Compare that with China, where 75 percent of toddlers are in school. Or Germany and Britain, where more than 90 percent of kids are.Once our poor kids fall behind, the chances are high that they will never catch up — often ending up dropping out of school and working low-paying jobs.In their book, education scholars Ajay Chaudry, Taryn Morrissey, Christina Weiland and Hirokazu Yoshikawa cite a growing body of research that's been following children since the 1940s. It shows that children who don't attend formal school until kindergarten start off a year behind in math and verbal skills.Parents who can't afford structured preschool often leave their kids with a relative or other informal child care providers. We respect the comfort, convenience and tradition of such arrangements. And we understand why half-day programs for 3-year-olds don't work for some working parents.But we also see the data — and worry that, too often, children who don't receive the right focused development at an early age lose out in the long run. The United States has fallen behind other countries."The earliest years are the most promising for brain and skill development, yet it is when the U.S. invests the least," Yoshikawa, an education professor at New York University, told the Washington Post.The book's authors make a compelling case that the United States should devote millions more for early education programs — getting all kids in by age 3, providing more help for affordable child care and working with poor families as soon as a child is born.The United States has to do better. This country can't afford to leave thousands of kids behind.