My crew and I were just talking about this yesterday.
I have to admit I am still shocked every-time someone asks me to visit the flightdeck during the flight. Hello people!!!!
One of the crew memebers yesterday said she was abused by a pax not so long ago when she explained to him the reason his son could not visit the flightdeck. "But my son is not a terrorist" was his reply.
WE tend to let kids have a quick look in the flight deck (time permitting) as they are disembarking.