Force sub-categories to use the parent category template

function pmq_subcategory_hierarchy() {
    $category = get_queried_object();

    $parent_id = $category->category_parent;

    $templates = array();

    if ( $parent_id == 0 ) {
        // Use default values from get_category_template()
        $templates[] = "category-{$category->slug}.php";
        $templates[] = "category-{$category->term_id}.php";
        $templates[] = 'category.php';
    } else {
        // Create replacement $templates array
        $parent = get_category( $parent_id );

        // Current first
        $templates[] = "category-{$category->slug}.php";
        $templates[] = "category-{$category->term_id}.php";

        // Parent second
        $templates[] = "category-{$parent->slug}.php";
        $templates[] = "category-{$parent->term_id}.php";
        $templates[] = 'category.php';
    }
    return locate_template( $templates );
}

add_filter( 'category_template', 'pmq_subcategory_hierarchy' );