def define_child(name, index)
      define_method(name) do
        children.at(index)
      end
      private name
    end
def define_child(name, index)
      define_method(name) do
        children.at(index)
      end
      private name
    end
Define named child

 @param [Symbol] name
 @param [Fixnum] index

 @return [undefined]

 @api private