1. override
String debugDescribeChildren(String prefix)

Returns a string describing the current node's descendants. Each line of the subtree in the output should be indented by the prefix argument.

Source 

@override
String debugDescribeChildren(String prefix) {
  if (child != null)
    return '$prefix \u2502\n${child.toStringDeep('$prefix \u2514\u2500child: ', '$prefix  ')}';
  return '';
}