Does Family Link work on tablets that are sold with Andriod 6.0 then get software upgrades to 7.0? Google Family Link requires Android 7.0 Nougat on most devices.

Does it work on tablets that are sold with Android 6.0 Marshmallow then get software upgrades to 7.0?

And if so, what is the sequence I need to follow to make this happen?

Edit: Family Link instructions require a clean device / factory reset before logging in the child. If I log in, then upgrade to 7.0, then factory reset, will I still be on 7.0 after the reset? Or is there some other way to make this happen?
Yes, an app that only works with Android 7.0 will work on all compatible devices regardless of what OS the device first shipped with.


  Children can run Family Link on the following Android devices:
  
  
  Nougat 7.0+  Compatible with all devices. (Source)
  


So the sequence to follow would be,


Make sure that the device you are purchasing, has an update available for Android 7.0
Update the device to latest software from settings  &rarr; about device  &rarr; software update.
Download Google Family Link, and set it up. 


To answer your last question:


  Your phone does not keep an original OS image. Thus, once you update
  your OS (either by OTA updates or by installing a custom Rom), you
  will not be able to revert to the older Android version. Doing a
  factory reset should just reset the phone to a clean slate of the
  current Android version. (Source)