I let passengers visit my flight deck all the time. No security problem. You just have to know when to shut down the visit due to checklist management before push, etc. Anytime I tell them time's up and they've gotta' go back to their seats, they are always extremely polite and thankful.
Honestly, sometime we all take this security thing a but too far. By the time a passenger reaches the aircraft, they have been screened 3-5 times. Nothing to worry about there.
If a kid comes up and his parents are there I'll put him in my seat and let Dad get a photo of him. Tomorrow's aviators, folks. Someone's gotta' fly us around when we retire.
Sometimes, a little
PR goes a long way to elevate our profession.