Hello Rose
I agree with your son. I went to Vietnam and Cambodia with JY in 2008 and we visited both S21 and the Killing Fields.
The whole experience is VERY distressing. You can read about the atrocities but you cannot even begin to imagine the real horror of what went on until you are told by someone who was caught up in it all and see things for yourself, and, as your son has said, it is something everyone ought to see, however distressing it is, as that is the only way you can really understand what happened there.
For me personally I am glad I went to both S21 and the Killing Fields, but I can fully appreciate it might be too distressing for some, and as it is built into the itinerary some may want to opt out, by highlighting it as OPTIONAL (if I am understanding it correctly) it is a good way of making it clear you can do so.
The more experienced of us will know that within reason you are never made to do anything other than move on to the next location, but any first timers may not be fully aware of that and feel they have to go along with everyone else, although on this occasion I would think a good TM will fully explain the situation.
Have a great holiday
Avocet