法典基本上解释了如何做到这一点：

if ( $category_to_check = get_term_by( \'name\', \'fruit\', \'category\' ))
   post_is_in_descendant_category($category_to_check->term_id);

https://codex.wordpress.org/Function_Reference/in_category#Testing_if_a_post_is_in_a_descendant_category

if ( ! function_exists( \'post_is_in_descendant_category\' ) ) {
    function post_is_in_descendant_category( $cats, $_post = null ) {
        foreach ( (array) $cats as $cat ) {
            if (!absint($cat)) {
//            $cat = get_category_by_slug($cat); // option one
              $cat = get_term_by(\'slug\',$cat,\'category\'); // option two
//            $cat = get_term_by(\'name\',$cat,\'category\'); // option three; by category name, not slug, as in the Codex
              if(!$cat) return false;
              $cat = $cat->term_id;
            }
            // get_term_children() accepts integer ID only
            $descendants = get_term_children( (int) $cat, \'category\' );
            if ( $descendants && in_category( $descendants, $_post ) )
                return true;
        }
        return false;
    }
}