Bob Ward, Naval Institute Press, 2005
No time to read the whole book, but has two passages which clearly describe Wernher von Braun's developing interest in rocketry. On page 11 is his 1924 "experiment" with a rocket-powered wagon at age 12:
... In 1924, he got an irresistable idea, probably inspired bt the experimental rocket-powered automobiles then making the headlines. ... The two older boys bought six large skyrockets and lashed them to Wernher's coaster wagon ... After wheeling the wagon into Tiergarten Strasse, Berlin's most upscale street, the boys lit the fuzes, and Wernher hopped aboard for the ride.
Von Braun later painted a vivid picture of the ensuing chaos. ...
reference: Wernher von Braun, "Space Man - The Story of My Life," American Weekly, July 20, 1958 (part one of a three-part series), 8.
On page 12:
At age 15  he read a science fiction article in an astronomy magazine that ignited his zeal to make rocketry and space exploration his life's work. "I don't remember the name of the magazine or the author, but the article described an imaginary trip to the Moon. It filled me with a romantic urge. Interplanetary travel! Here was a task worth dedicating one's life to! Not to stare through a telescope at the Moon and the planets, but to soar through the heavens and actually explore the mysterious universe! I knew how Columbus had felt."
reference: Daniel Lang, "A Romantic Urge," New Yorker, April 21, 1951, 75-93.
Later that year, Wernher was reading a pamplet on astronomy and saw a drawing of a rocket speedong through space toward the Moon. It illustrated an article about a man who would soon become his mentor, rocket theoretician Herman Oberth ... Intrigued, Werner sent for a copy of Oberth's classic book, The Rocket into Interplanetary Space, which had been published two years earlier [actually in 1923, perhaps this refers to a later edition, but before Ways...]. He was soon "shocked to discover that it contained mostly mathematical equations." A resolve that would come to characterize him emerged. Wernher, who disliked math, recalled much later. "I decided that if I had to know about math to learn about space travel and rocketry. then I'd have to learn math."
reference: Time, February 17, 1958 cover story, "Reach for the Stars," 22.