<?php

add_action ( 'woocommerce_order_status_completed_notification', 'eg_maybe_remove_order_status_completed_notification', 5 );

function eg_maybe_remove_order_status_completed_notification( $order_id ) {
	global $woocommerce;

  if ( $order_id && class_exists( 'WC_Subscriptions_Order' ) && WC_Subscriptions_Renewal_Order::is_renewal( $order, array( 'order_role' => 'child' ) ) {
		$mailer = $woocommerce->mailer();
		remove_action( 'woocommerce_order_status_completed_notification', array( $mailer->emails['WC_Email_Customer_Completed_Order'], 'trigger' ) );
	}
}