As stated above, I also get asked on average, probably once or twice a week, if a child + parent etc can visit the flight deck in flight. I always politely decline and explain the reasons why it isn't allowed. They are always very understanding. I always do my very best to speak to the PIC and ask if the child can have a look in the Flight Deck upon landing though. I then go back and tell the parents that if they remain behind after everyone has disembarked our lovely pilots will be happy to show them around the F/D. They are always very excited and accept the invitation every time.
Thus, there is never a problem
.