As StoneyB suggests in a comment above, the striking capitalization style that A.A. Milne used in his stories and poems about Christopher Robin and Winnie-the-Pooh (which appeared in four volumes across the years 1924, 1926, 1927, and 1928) was very likely the inspiration for a generation (or more) of children’s books to use initial caps for emphasis. A few examples from the Winnie-the-Pooh oeuvre, courtesy of Winnie The Pooh Quotes:
“Don’t underestimate the value of Doing Nothing, of just going along, listening to all the things you can’t hear, and not bothering.“
Would you mind coming with me, Piglet, in case they turn out to be Hostile Animals?”
”Then would you read a Sustaining Book, such as would help and comfort a Wedged Bear in Great Tightness?”
The claim of Sellars & Yeatman’s 1066 and All That to such credit (or responsibility) is undercut by the fact that it was published in 1930.
Another (and even earlier) serious student of the capitals-for-emphasis school was George Ade. A representative paragraph from his work is this one from People You Know (1903):
Once there was a full-sized Girl named Florine whose Folks kept close Tab on her. Any night-blooming Harold who presumed to keep the Parlor open after Midnight heard low Voices in the Hallway and then a Rap on the Door. If Florine put on her Other Dress and went to a Hop then Mother would sit up and wait for her, and 1 o'clock was the Outside Limit. Consequently Florine would have to duck on the Festivities just when everything was getting Good. Furthermore she would have to warn Mr. Escort to behave himself when they drew near the House.