On a flight back from MIA to LHR on 747 3 years ago my family were in the UD and when we boarded the cockpit door was open so I took my oldest son up and we had a look a couple of steps away from the door so as not to be intrusive. The captain spotted us and invited us both in for a closer look and the FO let my son sit in his seat for a few minutes as well.

It's probably my highlight on BA, the way it wasn't an issue and they were so happy to spend a few minutes talking to us.