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Driver dropped kid off outside school, with the car being stationary for a only a few seconds. A few seconds is enough to make them liable for a penalty so it seems.

Indeed it is, because it is a No Stopping contravention, and what's more, can be enforced by CCTV, as one of only a few situations where parking can be enforced in this way.

Street View also do not show any signage for this particular "contravention" nor any CCTV nearby which leads me to believe that this was a recent implementation

The "signage" is there, being the zig-zag markings and text.  There is no legal requirement for any CCTV signs.  The markings were there in 2008 so this is not a recent implementation. What is possibly new is using CCTV to enforce it.
https://maps.app.goo.gl/4fZrLUtDAHXjiin56

I suggest you submit representations about the delay in you receiving the PCN. They should re-offer the discount. Please also post all sides of the PCN so we can check it for fatal errors which could make it void.

But there's nowhere for this tidal wave of cars to go at this point and therefore IMO drivers are effectively contriving matters such that they stop because they have to and what looks like a perfectly able and fit looking teenager in this case alights as planned.

If not for the alighting a de minimis argument could be advanced with some prospect of success IMO, but if an adjudicator sees the video I think the written argument might not be enough.

The only way to get a decision one way or the other is for the OP to take them to London Tribunal, emphasising that the car was stopped in traffic as is very clearly shown, due to a vehicle ahead turning right. A passenger then took this opportunity to exit the vehicle.

Of course the full PCN penalty would be in play.

Did the child get out on their own or did you give permission.

OK OP, cutting to the chase...

Let me suggest that you were there as a matter of common practice with a passenger who you were taking to school as you do regularly. So, had you not stopped at the precise location in this instance, where would you have stopped in order for your passenger to alight? There's nowhere ahead on the left of the road, it's markings up to the dead-end.

So, what was your routine and intention on the day? There could well be a defence in your answer.

So what were the driver's intentions?

There might be a defence hiding in plain sight if only the truth was told. I'm not going to concoct a story, but if all the facts were given then perhaps a strong case could be made.

And there you have it.

My son attends *** school and I take them and pick them up each day. I am aware of the restrictions in Norlington Road itself and my regular practice, along with many other parents, is to drive past the school, turn right into ***, let my son alight on the waiting restrictions - in practical terms maybe a 10-second stop- and then return via Norlington. Seeing other parents' cars outside the school is common. Some ignore the markings and when doing so block the road for others behind who are forced to stop and wait. I would hope that no-one is foolish enough to do this more than once because the council's cameras are able to catch and penalise such practice.

And this is the central point in your defence. The council does not - I'd stake money on it- issue PCNs to cars simply ecause they're forced to stop but only those that do so with intent, even more so when their loved one(s) leap out from their cars.

My situation was no different on the day of the contravention than on hundreds of other days except that my impatient son decided to get out of the car at this point.

I hope you can see that the council's threshold for issuing PCNs is not simply being stopped- if it was then they'd run out of blank PCN forms rapidly- it's when they decide that an offence has occurred and that 'beyond reasonable control' does not apply.

I submit that this is my case. I had no intention of letting out my son at that point but instead doing what I do every other day. The mere fact that there was a hold-up and traffic was stationary is not, as I've set out above, the council's test, presumably it's intent and their only metric is: did a passenger alight? 

In my case a passenger did alight, but for the reasons given a contravention did not occur.

Is what I was getting at.....

Do the facts fit?