Unfortunately, despite my friend's personal experience and stories (like this one) with similar anecdotes, it is not official TSA policy to keep kids out of naked scanners:
If your child is able to remain standing in the required position for 5 seconds, he or she may be screened through the advanced imaging technology. If a child 12 and under goes through the machine and alarms, they have an opportunity to go through again or the TSA officer may use other procedures to resolve the alarm to reduce the need for a pat-down.
You may not be screened by this technology when carrying an infant or child.(You may recall that they changed policies for the under-12 set 4 years ago due to bad publicity, including a "modified" pat-down and the above-stated ability to go through the scanner a second time.)
It may very well be unofficial policy to put kids through metal detectors with their parents, and - just to keep us on our toes - they won't acknowledge it. But, it may also just be at the discretion of the supervisor(s) at a given airport.
For a large hub, with several security lanes - some with and some without naked scanners - it is probably quite easy to pick and choose who goes where. The randomized PreCheck may also increase odds if airlines can get families onto the list ad hoc. I do wonder what experiences are like at small airports with a single security lane (such as my hometown airport). What have you experienced firsthand while travelling with children?