To refund a payment that is paid through Sandbox you would go to the child’s profile or the Billing tab > Overview and go to the payers account. From here, click the down arrow > Click on the payment you wish to refund > Deselect any invoices it is applied to > Click Save Payment. Then click back on the payment and you will notice that in the bottom left hand side there is now an Issue Refund rectangle button. Click on the Issue Refund button > Add any comments if you wish > Click Save Refund. This will then put the money back in that payers card/account. 

Note: Do not click the Issue Refund plus button outside of the payment. This is intended for manual payments, such as cash, check, etc. This will not refund that payment.