My understanding is that as far back as the 15th century, long before we had processed cereal, that the settlers boiled various grains when they were really hungry. Raw milk has just enough butter fat in it to help stave off hunger. I imagine that some mother who had several hungry children & NO good food boiled up some grains & then added a little bit of raw milk in hopes that their babies would be satisfied until she could cook them a healthier meal.
The reason I am so sure about Frosted Flakes was that for a period when I was 10–11 the cereal ran a promotion in which each box of flakes contained a divisional shoulder patch from a US army or army air force unit operational in WWII. I went crazy collecting the things and trading the duplicates. I had a wonderful poplin jacket that I covered front and back with those patches which I stitched onto that jacket myself. To this day, I can still name the units belonging to those patches, and when the promotion ended, I would visit the army surplus stores to further enhance the collection. My favorite to this day is the Combined Operations Patch of the CBI (China Burma India) theater. The thing is big. It’s brilliant gold on a royal blue field depicting twin minarets with a thompson submachine gun spanning the 2 towers and an eagle with outstreched wings stretching the width of the patch perched on the gun. Absolutely gorgeous!
Many things were put on porridge years ago. Jam, honey, gravy; anything like that. And porridge is made from wheat, rice, flour. Anything that is stirred into boiling water and thus cooked.
I just poured milk on my weetabix & am happily munching away as I read this question.
I must admit however that I am oblivious as to how the trend began, just know it tastes divine.
Sanitariums in those days were largely for people recovering from or exposed to TB so there was a health factor to the diet. See or read “The Road to Wellville” by T. Coraghesan Boyle.