def valid_child_types(parent)
      # could be a node object, or maybe a string
      str = parent.respond_to?(:entity_type) ? parent.entity_type : child
      self.down_taxonomy[str]
    end
def valid_child_types(parent)
      # could be a node object, or maybe a string
      str = parent.respond_to?(:entity_type) ? parent.entity_type : child
      self.down_taxonomy[str]
    end
access the appropriate child entity_types for a particular parent or parent entity_type
 @param [Taxonomy::Node, String] parent the parent object or entity_type string
 @return [Array] an array of strings which are the valid child types for the child