The Wordfence extension enables you to trigger scanning process on child sites directly from your dashboard. To do this,
Go to the Wordfence Dashboard page (MainWP > Extensions > MainWP Wordfence Extension )
In the upper part of the page, you will be able to find the Start a Wordfence Scan button
This button will start a scan process on all child sites.
If you want to scan just an individual site, locate the child site in the Wordfence dashboard and click the Scan Nowaction link.
After the scan process is done, you will be able to use Show Results link to see the results.
Click it and the new Scanning Results tab will open for you.