WooCommerce: If a parent category has only one subcategory then it will automatically redirect to that subcategory.

woocommerce-single-category-redirect.php

<?php
	add_action( 'template_redirect', function () {
		global $wp_query;
		if ( is_archive() ) {
			$queried_object = get_queried_object();
			if (
				isset( $queried_object->taxonomy ) &&
				isset( $queried_object->term_id ) &&
				$queried_object->taxonomy === 'product_cat' &&
				count( get_term_children( $queried_object->term_id, 'product_cat' ) ) === 1 ) {
				wp_safe_redirect( get_term_link( get_term_children( $queried_object->term_id, 'product_cat' )[0], 'product_cat' ), 302 );
				exit;
			}
		}
	} );