Well markflyer that may be your view and you may be a student, but I am afraid it is a naive viewpoint. A Captain is in charge of an aircraft, its crew and passengers and any cargo. However that charge is expected to be conducted within the rules and regulations as laid down. There may well be occaisions when that has to be modified to suit a situation but it is not a matter of "do as he pleases" and of course nor should it be. Captains are not placed in their post by divine right, they are placed there by virtue of their demonstrated ability to not only fly the aircraft well, but to be effective managers in that role. Being an effective manager is seldom demonstrated by adopting a pick and choose version of the rules and constraints that one operates by. Indeed it sets a poor example to others and would do little to endear respect from other people.
Your bold type bit about people in the terminal doing their job is also a little naive and rather overstating the obvious in an ideal world.
If you are fortunate enough to be a Captain one day it will be because you are someone in whom your employer can place their trust. Allowing kids or anyone else on the flightdeck is constrained at this time in the Uk to pre flight and post flight only. As such it is not up to you to decide to violate this rule, without placing others in an invidious position, and thus demonstrating poor leadership. That is the point of the thread starters post.
Whatever you may feel, your version of common sense is not necessarily an adequate substitute for the statutes we currently have to work under. Should you one day find yourself in this position you will realise the point and better understand the difficulty expressed by tiggerific.