Let me just put an 'observation' here. I've heard a good few captains complaining about 'near-misses' with kids trying to fiddle with the centre panel controls etc., so they (the flight deck) have to keep their eye on them quite closely. If they're busy, it creates extra workload that they don't really want, especially when they're tired. I think that's what they worry about - uneventful quick visits seem to be OK, but it does increase the flight deck's workload especially when the visitors are kids. I know that cabin crew work very, very hard, but I hope you can understand that the flight deck can be under pressure - if the kid (or adult) does something wrong, it's still the captain's responsibility. At the end of the day, the flight deck, in particular the captain, always risks losing his job every time he goes to work - and things easily go wrong. Many just want to reduce the risk of something going wrong.