Instead of Object Space

module ConcernedParent
  module ClassMethods
    def children
      @children ||= []
    end
    def inherited(cls)
      children << cls
    end
  end
  def self.included(cls)
    cls.extend(ClassMethods)
  end
end

class Parent
  include ConcernedParent
end

class Child1 < Parent
end

class Child2 < Parent
end

puts Parent.children