This will add child subscription payments to a given subscription

give-recurring-dummy-subscriptions.php

<?php
/**
 * Create Recurring Dummy Subscriptions
 */
$subscription = new Give_Subscription( 68 );

$args = array(
	'subscription_id'       => 68,
	'amount' => 20,
	'transaction_id'  =>  md5(uniqid(rand(), true)),
);
$subscription->add_payment($args);
$subscription->renew();