child_class.rb

require 'nokogiri'

class ChildClass < Nokogiri::XML::Document
  attr_reader :whatever
  
  def initialize(whatever)
    @whatever = whatever
  end
end

ChildClass.new(500) # ==> no implicit conversion of Fixnum into String
ChildClass.new('X') # ==> wrong number of arguments (given 0, expected 1)

document.rb

# This file is part of Nokogiri

module Nokogiri
  module XML
    class Document < Nokogiri::XML::Node
      # ...
      
      def initialize *args
        @errors     = []
        @decorators = []
      end
      
      # ...
    end
  end
end

node.rb

# This file is part of Nokogiri

module Nokogiri
  module XML
    class Node
      # ...
      
      def initialize name, document
        # Empty
      end
      
      # ...
    end
  end
end