To email invoices you can either go to the Billing tab > Invoices > Select the dot beside the invoice(s) > Click on the top drop down (3 dots) > Select Email Invoices. 

Or, you can click on the invoice (in Invoices or Overview) > Click Email on the top right hand corner. This will then send out an individual email to each payer with their invoice attached as a PDF. 

If you have Parent Portal set up and the parents have access to their account, the invoices will automatically be updated to their portal for them to view at any time. This could eliminate the need to email them out.