Has drinking Alcohol always been seen as an "adults only" hobby, or have there been societies in history that would allow children into pubs/establishments and serve them, or which normalized giving alcohol to children in familial and social settings?
To be clear, I'm not talking about weak beer or slightly alcoholic drinks where fermentation occurred to kill off bacteria and to improve water quality. I'm specifically curious if it was ever considered OK to provide alcohol to children for the purpose of them getting drunk, to the point where it was a common occurrence?
You seem fairly familiar with the common practice of giving smaller children beer before potable water was widely available, and the idea of "drinking water" would have seem foreign to adults and children alike.
There is another practice that might interest you, which involves giving a baby a small amount of brandy or whiskey to stop it from crying and make the infant sleep more soundly in the night. The alcohol was often dabbed on the baby's lip, put into their milk or on the tip of their bottle. This is a folk medicine practice that has been common in many cultures that continues, in some places, to today.
What your question seems to be getting at, however, is the stigmatization of alcohol and the connection between "getting drunk" and adulthood. There would obviously be a different answer to your question for every single chronological and geographic framework that might interest you. However, in general, it might be interesting to consider the types of terms you are using and the distinctions you are drawing.
For example, has "alcohol" always been a universal category? In our culture, we are able to easily define what constitutes an alcoholic beverage, but this is not a historically universal definition. For example, in some cultures, there might be fermented substances that were given to children and adults as part of religious or spiritual rituals that had the effect of "getting them drunk", but that did not have the same connotations as being an "alcoholic" or "adult" beverage.
Another term to consider is the idea of "getting drunk." You say you are interested in situations where it was "OK to provide alcohol to children for the purpose of getting them drunk." Yet you rule out situations where children were given weaker beers. Consider the effect of even a weak beer on a child. You are right in saying that water was not a commonly consumed substance because of potability, but there are examples of children being given beer (or babies being given brandy) to make them go to sleep earlier, while adults would drink caffeinated beverages like tea, coffee, or hot chocolate. The effect on the children is essentially getting them drunk, and the intention of the adults would have been to make them drowsier so they would behave better and go to bed earlier.
TL;DR: The historical conceptions of what constitutes "alcoholic beverages", what the "symptoms" of drunkennness are, and why a person would want to be drunk or want their child to be drunk, are all shifting definitions that would be very specific to a certain social, cultural, and historical context.
A cursory glance of the scholarship on the history of alcohol and alcohol culture shows that much of the research tends to be localized to a specific country or region, so it might be in your best interest to decide if there's a specific area of time period that interests you so you can focus your reading.
Some sources that stuck out to me:
Iain Gately, Drink: A Cultural History of Alcohol, New York: Gotham, 2008.
Eric Burns, The Spirits of America: A Social History of Alcohol, Philadelphia: Temple University Press, 2004.
Altering American Consciousness: A History of Alcohol and Drug Use in the United States, 1800-2000, ed. Sarah W. Tracy and Caroline Jean Acker, 2004.
Thomas Brennan, "Towards the Cultural History of Alcohol in France," Journal of Social History, Vol. 23, No. 1 (Autumn, 1989), pp. 71-92
Robin Room, "Alcohol, the individual and society: what history teaches us," Addiction, Volume 92, Issue 3 Supplement 1.
Theresa D. O'Nell and Christina M. Mitchell, "Alcohol abuse among American Indian adolescents: The role of culture in pathological drinking," Social Science & Medicine, Volume 42, Issue 4, February 1996, Pages 565–578
Akyeampong E. Drink, Power, and Cultural Change: A Social History of Alcohol in Ghana, c. 1800 to Recent Times. 1996.
David G. Mandelbaum, "Alcohol and Culture," Current Anthropology, Vol. 6, No. 3 (Jun., 1965). (This source is a little dated, but it appears to apply a macrohistorical, anthropological lens you might be interested in.)