Advanced Uptime Monitoring extension uses 3rd party services to check for uptime, and they are the ones sending out emails when the site goes down.
On the other hand, if you are getting email notifications from the MainWP Dashboard site, that must be from the Basic Uptime Monitoring service.
This feature can be turned on/off globally in the Settings page, but it can also be configured per site, by going to the Child Site Dashboard > Edit page.