In my opinion - ALARM BELLS ARE GOING OFF!!! In my head there is no reason a 5.5 year old should KNOW this. he was shown it, seen it or experienced it.
This is NOT acceptable behavior. At this point - call your pediatrician and tell them what is going on. Get your son to talk to you - not accusingly about this "thought" and see where it leads.
Ensure you are talking WITH your kids about personal space, privacy and the no touching - if they have been touched - they can tell you ANYTHING. If they see or sense panic - especially in your eyes - they will be quiet - but if, say, you are eating dinner - say something like - where did you learn the game you were playing today? let them talk - no matter what they tell you - DO NOT PANIC! I know that's hard (as I had a teacher call my son stupid once and I should've killed her..it took EVERYTHING in my power to just say "oh, really? tell me more....got as much information then called the school and said OVER MY DEAD BODY"...) any way - get them to talking...listen.
Make an appointment with the pediatrician ASAP. Watch them, especially him...and let them know that NO ONE is to touch them in their private parts, they can tell you if something like this was to happen - there are NO SECRETS...
Best of luck!! You will be in my thoughts and prayers!