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 p****** p****, 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!