This is the new normal, and we had better get used to it. I am sorry your child was upset, but given everything that's going on in the world today, we have to teach our kids to expect this sort of thing - and not just at airports, either. Sporting events, parades, theme parks - anywhere large crowds gather - are considered to be targets now. We cannot exempt children from searches for the reasons already articulated here. We can be outraged at the situation and the circumstances that brought us to this point, but we cannot be upset at the actions being taken to try to safeguard us. My DD has wanted to be wanded everytime she has traveled, and ironically, has never been selected. She is very fascinated by the gadgets they use! I travel an awful lot, and I get pulled every 10-20 flights or so. One way tickets seem to be the fastest route to the enhanced security line! One of the ways to keep from getting selected is to get a frequent flyer number and attach it to your record. It helps to create your traveler profile and you'll be less likely to be randomly picked. Basically, there are triggers for selection, but mostly it's a computer thing, and the computer has no idea how old the traveler is.
The best thing though is for you to have explained the possibility of this to your child ahead of time. And if your child is selected, for you to remain calm. The other day I watched as a woman voiced her outrage and displeasure that her child had been selected, all the while the kid is standing there watching mom look like a lunatic. The search would have been over in 2 minutes but instead bc she argued with them, the kid had to stand there for 10 mins while they got the supervisor. Now that would upset anyone!
Last, crisi, while I do not dispute your account of your friend's run-ins at Security and the existence of a list somewhere, I'm pretty sure TSA does not conduct cavity searches in US airports. They don't have the legal authority nor, from a practical standpoint, anyplace to do this in most places. Was your friend traveling internationally? Because INS & Customs do have that authority...