code: def child_isinstance(block, child_id, block_class_or_mixin):
      
      def_id = block.runtime.id_reader.get_definition_id(child_id)
      type_name = block.runtime.id_reader.get_block_type(def_id)
      child_class = block.runtime.load_block_type(type_name)
      return issubclass(child_class, block_class_or_mixin)
docs: Efficiently check if a child of an XBlock is an instance of the given class.
  
      Arguments:
      block -- the parent (or ancestor) of the child block in question
      child_id -- the usage key of the child block we are wondering about
      block_class_or_mixin -- We return true if block's child indentified by child_id is an
      instance of this.
  
      This method is equivalent to
  
          isinstance(block.runtime.get_block(child_id), block_class_or_mixin)
  
      but is far more efficient, as it avoids the need to instantiate the child.