We know a lot about the ideal environment for a happy whale or a happy mountain gorilla. We’re far less clear about what constitutes an ideal environment for a happy human being. One common measure for how clean a mountain stream, is to look for trout. If you find the trout, the habitat is healthy. It’s the same way with children in a city. Children are a kind of indicator species. If we can build a successful city for children, we will have a successful city for all people.