The practice shows up all over Latin Europe in various ways and elsewhere too, I assume. Children routinely have been introduced to wine at dinner with water dilution, in modern times. Adding a little wine to "doubtful" water (as a germicide or whatever), whether or not effective, is a folk technique, ancient I suppose. The cultures that have drunk wines routinely for eons also tend not to hold it sacrosanct, so that you see casual mixing of (ordinary) wines not just with water, but in suitable circumstances with carbonated water, fruit juices, soft drinks, or other flavorings (cassis in a kir is famous).
Saintsbury in his classic anglophone beverage guide _Notes on a Cellar-Book_ treated the mixed beverages so customary in his native Britain, though some were fading in popularity when he wrote (1920). Sherry cobbler with lemon, sherry and seltzer, negus, flip, punch, cups, "bishop," "Cardinal," and "Pope" (mulled Port, Bordeaux, and Burgundy respectively -- though "No burgundy is really suitable for mulling, while to mull good burgundy is a capital crime"). Sainstbury grew up drinking beer, wine, and fortified wines, seemingly unafflicted by fashionable prejudices of what was "considered" good to drink, and went about trying all sorts of things and judging by how they tasted. He confesses adding sodium carbonate to a "modern" Marsala to touch up its pH and make it more agreeable. (His accounts of encountering outstanding cask ales, in country inns during walking holidays and so on, become poetic.)
I don't know where the original poster is located, but one of the several quirks of North American wine consumption is that the beverage tends to be put on a pedestal. Fussed over and served intacto. (Unless the family has been accustomed to wine for generations, which is the exception and not the rule.)
-- "Some years after I had invented it [a recipe for a `cup' or punch that was very popular with party guests] I gave the receipt in an article in the _Saturday Review,_ which used at that time to confide to me most books on eating and drinking. Before long it began to appear in such books themselves, as indeed I had altruistically expected, for they are almost inevitably compilations. But the gradation of titles was very amusing. The first borrower honestly quoted it as `Saturday Review Cup'; the second simply headed it `Another Cup.' But the third trumped both them and me, for with a noble audacity he (or she, as I think it was) called it `My Own Cup.' " -- Saintsbury. [The cookbook business, in a nutshell. Actually not just cookbooks. -- MH]