Fly from Denver to Dickinson... besides the fact that there is no toilet for a 2-hour flight there is also no cockpit door! You can see everything..and after having seen two guys younger than me (and I am only 23) land a plane, I was close to take the Greyhound for the next trip!

I would think that you have a larger chance of finding pilots bending the rules on smaller routes and non-US carriers, but I would also dare to try it after landing during deboarding. Or hint during boarding that your kids really would like to see this and ask if the pilot wouldnt mind let them look in the cockpit after the flight - and see if he even invites them during the flight.