Parent Table checkbox changed, triggers automation that updates child table for notification

I tested this once out of the curiosity of that exact question, and the Automaton ran slow enough to be seen as separate updates and hence separate email Notifications. 

... and the release notes just came out, so my answer has a best before date of October 14th! In other words, I retract my answer.

• Starting in November, the modify records action in an Automation will now edit records in bulk instead of one at a time. This will make your automations significantly more efficient and prevent some �rate threshold limit� errors from happening when you have an automation that triggers another automation. Note: If you are using a modify action to trigger email notifications with a custom message, you should make sure the notification is set up to properly use the multiple record version of the email notification. More about setting up multiple record notifications.

https://www.quickbase.com/blog/october-2018-release-notes

I have not tested this or done something similar. However, I concur with Mark's 2nd comment that it will be a "multi-record" operation, but for a slightly different reason.

The "starting in November" release-note really does not directly answer your question. It seems to imply, however, that an automations with an action step that modifies more than one record will be processed like a "multi-record" operation. Possibly, the internal implementation is similar to doing a grid-edit on multiple records and saving them as a batch (not one by one). If so, then the internal implementation for your follow-up email-notification would be the same as if you had done a batch grid-edit or a csv-import, i.e., a multi-record email.

As a precaution, you might use the same email template for both single-record and multi-record scenarios, if you can craft the text of the email appropriately to fit both scenarios.