I wonder, in regard to the Scandinavian babies being left outside with exposed faces, but otherwise well-bundled, if there isn’t something to it that maybe isn’t an issue in other countries. If these are breast-fed babies, getting vitamin D may be difficult for them, because it isn’t in breastmilk, so they rely on sun exposure to get it, which on a baby’s face alone, in a place far from the sun, means extended exposure. Babies are born with a reserve of vitamin D, but they still need some exposure, or a supplement. (Formula, at least in the US, has vitamin D.)
I don’t know what people outside the US feed babies when they start eating other foods, but unless it’s fortified, it may not have much vitamin D either.
Now, I’m not saying that people are leaving their babies outside consciously for the purpose of getting their vitamin D, but it may be that babies who get out a lot, and stay out are healthier, and that has not escaped notice, hence the development of a custom that seems odd to us.
In the US, babies usually don’t get vitamin D deficiencies, because the period that they’re bundled up and kept indoors isn’t usually more than three months out of a year, and baby cereals, which most parents start by six months at the latest, have vitamin D, although in the last decade, there have been some reports of vitamin D deficiencies in babies who are breastfed, and then have sunscreen slathered on any time they are outside, because the PSAs about sunscreen on children have been so successful.
My son was breastfed, but I couldn’t quite keep up with him for the first few weeks, and he had trouble getting a good latch right away, so sometimes I had to pump and bottle-feed him breastmilk, the result of which was that he got some formula supplements-- about 4 oz. a day for his first six weeks. Then, he started eating a little cereal at four months, which was early, but he had a great desire to eat something, as evidenced by his attempts to try to take food off our plates and put it in his mouth, and he was really big for his age, so his doctor thought it would be OK. Anyway, it kept him from needing vitamin D supplements, even though he was a winter baby.