Pax cockpit access - are there rules?

[takemehome]

Folks, I apologise if this is in the wrong forum room. This is my first post, though I've been an interested SLF member here for many years and thus I value your sage opinions immensely.
Last night I flew Nok Air (DD), the budget arm of TG, from Krabi to the recently re-opened Don Mueang airport in Bangkok, and was surprised to see a civilian go into the cockpit while we were on the ground prior to departure. I mentioned this to the cabin crew, who told me it was OK because "he's a captain".
That was an acceptable explanation and I thought no more, until we had just started descent into Bangkok and he walked back to the cockpit with his two sons (probably around 14 and 12 y.o.) and took them in and closed the door.
At this point I became concerned, because we now had 5 people in a 734 cockpit, and the crew were obviously running through landing procedures.
I voiced my concerns to the cabin crew and asked them to pass them to the Captain, which they did, and this guy and his two kids emerged some minutes later. As he walked past me he smiled and handed me his business card, which had him as a senior employee of Nok Air.
Apart from what appears to be an abuse of his position with the airline, are there any international rulings about having passengers in the cockpit in these days of heightened security, or should I just put this down to idiocy, particularly as he waited until we had started descent to wheel in the kids? Or perhaps it varies by airline?
As we were collecting our baggage he came over to engage me in conversation about where I lived (unfortunately for him, in Bangkok) and what we'd been doing in Krabi. I told him straight out that I thought he'd acted irresponsibly, to which he replied that it was OK because his job is making sure the crew are working properly (though what that had to do with showing his two kids around an operating cockpit escapes me!)
I'd be interested to know if anyone knows the rules and regs pertaining to pax in the cockpit, particularly in Thailand where the rule of law seems to be discretionary at the best of times.
Cheers.