While this actually happened in a Female Restroom, an 8 year old girl was murdered in Australia in a shopping centre bathroom. It was in the middle of the day, on a weekend in a very busy centre. It happened in under 10 minutes. She was molested and murdered. We can say that it doesn't happen often (thank god!) but it DOES happen.
Well, it seems that if you are adamant about safety at any cost, your only choice is to keep your kids in diapers until they're 18 and never let them go into public restrooms at all. Your own constantly repeated example shows that using the women's room is not guaranteed absolutely safe, and your fears tell you neither is the men's room, so what choices have you left yourself? Probably the best is to not take them to WDW at all, in fact they should never leave the house at all.
There are no guarantees in life, but you do you child no service to teach them to live in fear. You take reasonable precautions, you take the probablities into account and plan accordingly, and you hope a lot. But you can't obsess endlessly about keeping them absolutly safe because they never will be. And you don't have the right to intrude on everyone around you, and make their lives uncomfortable in your chase after an illusion.
BTW, if you do need to change your 16 yo son's diapers, there are facilities to do so at the aid stations. Some people don't have the choice of raising independent children, you do.