The single biggest child site issue that you can end up dealing with is why is the site email not being sent. You could use an SMTP plugin and then use SMTP creds to connect. But using a transactional email provider makes it easier in knowing that your child site email will be delivered correctly. The delivery of site email can be a pain to deal with on child sites. If a site is using WooCommerce or a form plugin like Gravity Forms, you want to ensure that form notifications and customer order notifications are being sent out to customers.
SendGrid offers a free plan which provides 40,000 emails a month, which should be more than enough for most MainWP dashboard sites. Using a transactional email provider like SendGrid, SparkPost or SendinBlue. Go to https://signup.sendgrid.com/ After you have signed up for a free plan you will be in the main app. On your MainWP dashboard site look for SendGrid and install it as a new plugin on the site. Go back to the SendGrid app and then click on Settings > API Key then click on the Create API Key button. Create the API, then on the next screen the