Individual payer - go to the child’s profile or the Billing tab > Overview and go to the payer's account. From here,  click the down arrow in the payer's billing account > Click Add Payment on the left hand side > Choose Pay Through Sandbox. Then enter in the amount or select the dot beside the invoice to pre-populate the amount and apply it to the invoice. Click Save Payment when you are done. 

Groups of payers - go to the Billing tab > Invoices > Filter for: Pay through Sandbox, Unpaid and the Date Range - filter for the date you put for the invoice date. (eg. The monthly invoice for January 1 - 31 was dated January 1. So, filter for January 1 - January 1 to pull up those monthly invoices) Click Filter at the bottom of the drop down when you are done. Now select the payers in the list by clicking the check box on the top bar > Click the top drop down menu > Select Pay Invoices Through Sandbox. From here, all these payers will show up in a list where you can edit the amount you wish to charge otherwise it will pre-populate to the amount of the invoice. You can also change the day for which these payments will be processed at the top (Payment Date) and clicking Apply. If you want to process the payments for payers all on different days you are able to change the process date for just that payer. You would do this by changing the Payment Date within their billing section. When you are done, click Save Payments.