The argument that they are doing what they were asked to do does not make it right. It is a condoned sexual assault on a child - or, for that matter, on an adult.
One can draw a similarity between various regimes and their 'legal' treatment of people.
Now you can argue that it is a contractual condition of buying a ticket and flying that you submit to what is technically a sexual assault. Then there is a question as to whether or not such a condition is legal, and I believe the ACLU have ongoing legal actions about this. If the Supreme Court rules against the TSA, then there could well be a number of actions brought, and it would be very hard for a TSA staff member to say 'I was only doing my job' as a defence.