What Do I Tell My Son About School Segregation?

How do I explain this to Simon, the fact that large-scale neighborhood segregation too often goes hand in hand with economic deprivation. How do I tell him that it’s not that white people aren’t allowed, it’s that they are exercising their power to opt out? That Brown v. Board could only open school doors for black students, that it couldn’t keep white students from walking away? In other words, how do I explain the legacies of slavery and Jim Crow to a young child?