I don't have any personal "fears". We travel in an even numbered group and hardly ever go to DHS because it's our least favorite park so I really don't have to worry about it for me. It still made the hairs on the back of my neck stand up reading about it though. Maybe it's the teacher version of spidey-senses, but my gut feeling is that it is not a good idea to place kids in a ride vehicle alone with a stranger where they can be out of the sight of a parent or chaperone.
Why? Here is a list of inappropriate behaviors that children could encounter from being seated next to strangers on airplanes.
Link to article below.
http://www.airsafe.com/kidsafe/kidsolo.htm
I know that the ride is shorter than an airline flight, however, it doesn't take long for some of these inappropriate behaviors to occur. It is good to coach and instruct children on how to protect themselves and how to react to inappropriate behavior, but despite educational efforts to do so many children don't respond in the ideal way when faced with stranger danger. They are, after all, children. The best thing to do is to minimize risk.
Honestly, it's been awhile since I've ridden TSMM. Maybe the risk is not as great as I am imagining. Maybe you can arrange your family so a child is not alone or out of your sight. Maybe my "spidey-teacher-senses" are overreacting. I just believe that we should always be looking out for the safety and welfare of kids. Unfortunately, in today's world they greatly need it, even at the happiest place on earth!