class Parent(models.Model):

  child_class_names = (
    'Child1',
    'Child2',
    'Child3',
  )

  def child_object(self):
    for child_class_name in self.child_class_names:
      try:
        return self.__getattribute__(child_class_name.lower())
      except eval(child_class_name).DoesNotExist:
        pass
    return self


class Child1(Parent):
  pass

class Child2(Parent):
  pass

class Child3(Parent):
  pass