The ad request url generation code located at amphtml/extensions/amp-ad-network-doubleclick-impl/0.1/amp-ad-network-doubleclick-impl.js looks for a JSON parameter named 'tfcd' to copy as a url param named tfcd into the ad request.

However, DoubleClick and A4A documentation suggests that the name of the JSON parameter has to be fully spelled out as tagForChildDirectedTreatment, so I believe that the current code won't find any JSON parameters named 'tfcd' since publishers have been tagging their pages with 'tagForChildDirectedTreatment'.