functions.php

<?php

/**
 * Category archive query 
 *
 */
function be_category_archive_query( $query ) {
	if( $query->is_main_query() && ! is_admin() && $query->is_tax( 'product_category' ) ) {
		$ancestors = get_ancestors( get_queried_object_id(), 'product_category', 'taxonomy' );
		$ancestors[] = get_queried_object_id();

		$tax_query = array( array( 'taxonomy' => 'product_category', 'field' => 'term_id', 'terms' => $ancestors ) );
		$query->set( 'tax_query', $tax_query );
	}
}
add_action( 'pre_get_posts', 'be_category_archive_query' );