The weird requirement that everyone smile at cameras and photographers only take pictures of smiling people is a relatively recent cultural convention, started in the US and then spread around the world by US influence. I think it has something to do with the culture of photography used for advertising/sales.

If you try taking pictures of very young children who haven’t yet been indoctrinated into the smile-for-the-camera tradition, they’ll make a wide variety of facial expressions for the camera. But American (& al.) children learn very young that the cultural norm is to always make sure to smile when a camera is pointed at you. Even if the children don’t figure this out for themselves (and they usually do), parents and other adults are often quick to tell them to smile or scold them for not smiling.

If you look at pictures of people in posed pictures in the 19th century or in parts of the world where there are few cameras/photographs, the standard expression is usually a serious/formal one. And if you look at candid pictures, people will just be making whatever natural expression they had.

As a photographer, 80% of my time is consumed with removing the frozen into frigid mask smile from the faces of my subjects.

Natural emotions and thus facial expressions are fluid and involve things beyond our control.

A smile held for the camera accompanied with non-smiling eyes (very rarely can someone create a smile in the eyes and hold it) looks unnatural.

Maybe it's a cultural thing but I don't always associate smiling with "happy". My wife were looking at some old pics of our son and we commented how happy he must have been in one of them because of how serious he looked!

NB I'm in Scotland, which does have a bit of a reputation for being a bit dour. ;-)

Please dont't base your worldview on american customs. In some countries, doing a forced smile for photos just isn't a thing

I want to move there. Everyone assumes my lack of smiling means I'm not happy.

It's a cultural thing. They think smiling for cameras is weird.

The photos in the article are not shot in-the-moment. The children pose in front of a camera, probably with lighting around. They don’t smile because they are in a kind of serious setting, look into a camera and do not interact with anyone.

Edit: Maybe the photographer is bad at soliciting emotions from his subjects.

I dunno, I visited Iceland a few months ago and the people there seemed way happier than most people in my social circles here in Seattle.

In some cultures a fake display of emotion like a fake smile is consider a form of deception, not much dissimilar from a lie.

It's a cultural thing. A lot of cultures developed in cold high latitudes seem to discourage open expression of emotions for some obscure reason.

Now go to the mediterranean riviera and everyone is smiling broadly and talking loudly even when they feel like murdering someone.

Came here exactly to point out the stern faces of the kids... as noted it might be a cultural thing, but for me it immediately caught my attention.

The art director has a choice to accept or reject photos as they see fit. The art director can select a photographer by style, and ask the photographer to take pictures of any variety.

The photographer will direct the subjects to elicit fitting behavior for the shoot, even if mugging for the camera seems dishonest, a professional photographer will try to coax exagurated behavior from non-professional subjects.

That they are all aligned in the same style would indicate an editor's intent. The desire to convey a premise.

Perhaps:

  serious facial expressions 
  for a serious subject.

The kids wore appropriate clothes. The kids were posed. Had they blinked or crossed their eyes for any of the shots, more pictures would have been taken until a professional shot had been captured.

It's not unheard of for an editor to reject an entire photoshoot if it distracts from the content. They'll spend the money to get it done right, if it doesn't work.

If the subjects were all giving the middle finger, because it was normal in Iceland, do you really think that would have landed in the article?