IRL Kumis is a kind of alcoholic beverage made from fermented horse milk. The alcohol content is very low, only about .7-2.5% alcohol by volume. (Around half the content of your average beer at most). The drink was traditionally made by turkic and Mongolian tribes (whom the oni in this series are modeled after). The mongols would feed this drink to their infants and small children to keep them healthy as the bacteria used to ferment the kumis was antibacterial, sort of like a rudimentary penicillin. Which would lower infant mortality rates significantly as it would help protect them against minor illnesses.
@Alashara
It has evidence in their culture, children that don't get alchohol and instead only drink water are more often sick than those that do. Could be some correlation causation issue but the point is x causes y.
See my above comment about kumis, an IRL mongolian alcoholic drink made from fermented mares milk that could help prevent certain minor illnesses as a infant or young child