From the Appointments List page (not the Appointments page) in the Admin UI, you can search by a single date, a range of dates, or by an event. This will then bring up a list of parents with appointments for that date, dates, or event. 


On this page you can email participants, delete, confirm and recreate slots and appointments.


On the top of the page is the Send Email button, you can then select the option to Send to All or to Send to Selected. A dialog box will come up that looks like this: 

This will let you send a Reminder email or a Custom email to your group of parents. You can use the current email template or you can customize as needed. The template for both emails is located on the Settings / Notification Setup page.