The mother and child, seem very aware that metal detector screening is not an option. The mother and child also seem aware that AIT IS an option; from the facebook post "Instead of the typical smile and use of the other machine or swiping his hands for explosives..."

"Other machine", to me, suggest the AIT.

TSA does not prohibit children from utilizing the AIT. The son could have easily been screened via the AIT.

From TSA.gov

Implants & Internal Medical Devices

Inform the TSA officer that you have ... a pacemaker, ... .

Screening

Advanced imaging technology can facilitate your screening and reduces the likelihood of a pat-down. You should not be screened by a walk-through metal detector if you have ... a pacemaker. Consult with your physician prior to flying.

If you choose to not be screened through the advanced imaging technology or you alarm the walk-through metal detector, you will undergo a pat-down screening instead.

Advanced Imaging Technology

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.