【覚書】WP：親カテゴリのテンプレートを小カテゴリに！

functions.phpにこれをプラス

add_filter( 'category_template', 'my_category_template' );

function my_category_template( $template ) {
    $category = get_queried_object();
    if ( $category->parent != 0 &&
        ( $template == "" || strpos( $template, "category.php" ) !== false ) ) {
        $templates = array();
        while ( $category->parent ) {
            $category = get_category( $category->parent );
            if ( !isset( $category->slug ) ) break;
            $templates[] = "category-{$category->slug}.php";
            $templates[] = "category-{$category->term_id}.php";
        }
        $templates[] = "category.php";
        $template = locate_template( $templates );
    }
    return $template;
}

http://studio-freetown.com/post-783/

参考にしました。