Monitor Usage by Device

If my children are each setup with an xfinity profile and their individual phone, computer, TVs, etc. are assigned to each of them, how do I establish which child is using massive amounts of data?

Activity Tracking does not accomplish this.  The Activity Tracking is a joke and is entirely useless, incorrect, and misleading.  

As a parent, I need to know which child and which device(s) is being used to for massive amounts of data.

How can I monitor device usage, not profile activity?

Edit/Addition: I'm not asking to know what site/URL is using data, but what device is using data.  This should not be a privacy/legal issue as insinuated by xFinity response in another post.