I remember this distinctly from elementary school, but my 1980s edition had different, ominous illustrations, especially the one where John's mother is turning into chocolate.
John Midas (allegory!) is a boy who loves chocolate more than anything, and whose parents are upset because he eats so many sweets that he refuses his healthy, healthy bacon. The descriptions of healthy food in this book are quaint, but it's the 1950s and ultra-processed food was not the issue it is now. The allegory against candy is strong, but this book is still read in classrooms, and how do the kids who have pop at home as the standard beverage see the "candy in moderation!" didacticism? The world has gotten weirder, and John is allowed to walk all over his neighborhood cadging candy from his friends, because, along with the ultra-processed food, we had yet to fuck urban planning in 1952. The kids these days have a big hole to dig themselves out of.
Speaking of unbridled capitalism, John finds a magic store, and the proprietor (find an independently owned candy store in North America nowadays outside of a tourist town) sells John a box of chocolate, even though John has no money but the mysterious coin he has just found on the ground. A coin with a picture of a fat boy on one side and John's name on the other. This is the only actual potential fat shaming in the book, the implication being, presumably, that if John eats so much chocolate he will get fat. John takes the box home, eats the one magic chocolate, and the curse takes hold. Poor John.
I remember the water fountain scene from reading this as a kid. It was dark.
I don't think school night birthday parties were a thing in the '50s either, but the author needs to present a couple more turning-things-into-chocolate scenarios in there before John dehydrates and dies.
Patrick Skene Calling appears to be a British emigrant to Canada, so there are indicators that this book takes place on two continents, kind of subtly but at random.
Apparently there are multiple sequels that no one's ever heard of, including one where John visits the Australian dreamtime. Am ready for it.