(Chapter 102)
Rambled about this before on this post, but I really wish they would explained how the search party is able to see out of their masks, even if it was a single panel of Emma pointing out something like nearly imperceptible eye slits.
(Chapter 147 | Chapter 149 | Chapter 150)
Older, classical masks like those of the elder Bayons (compared to those of Bayon II's wife and children), Yverk, and Geelan's clan incorporate a pair of symmetrical, horizontal, and frequently vacant eye holes among their designs.
In the chapter 90 bonus sketch, this is mentioned as respecting tradition, which I interpret as a holdover from the days of demons hunting humans, and the intention of that incorporation was to play on pareidolia-enhanced fear among humans who, out of habit, would look toward the horizontal pair of eye holes first when gazing upon demons in combat and would frequently be treated to an unflinching, menacing abyss. It's a good subtle tactic to unnerve them before they take in the rest of the mask.
Once the farm system was instituted and demons weren't regularly warring with humans anymore, it became unnecessary and seemed to fall out of style among the general population who weren't banished like the Geelan clan.
I assume this is the main reason the kids don't opt for horizontal eye holes. There's also the issue of demons potentially catching sight of their human eyes, but there has to be some black cloth to work with in the bunker that could have lined the masks if they couldn't get whatever magic Bayon and Yverk use for no other eyes or skin to show through their horizontal mask sockets.
Either way, it's one less thing to make them stand out from the crowd when they're already contending with their comparatively short statures, human hands and feet, and having to wear perfume.
