I presume it was before the kids/strangers rules (which I have to tell you I find insulting) were introduced, but I once returned on BA Berlin to London and was put next to the two UMs (unrelated) on board.
On climbout I unwisely suggested to the 8-year old boy next to me that maybe a visit to the flight deck (I told you it was a while ago) could be arranged. Oh my, that was a mistake ! "Oh wow! Can I go now can I go now can I go now ???". I said I would ask the crew. "Look, here she comes, ask this lady ask this lady". And so on for the rest of the flight !