Seimei Metamorphosis by Akiko Moriya
Abe no Masuki's son, Kuzumaru (later Seimei), had the power to "foresee" people's deaths from an early age. Kuzumaru has been raised without leaving the mansion so that disasters do not befall him.
When he was seven years old, he met Tsukuge (Yasunori), a boy who was preparing to train as an Onmyoji, and Tsuru-kun (Hiromasa), the grandson of the late emperor.
Three people who become close because they are not familiar with people. However, because of Kuzumaru's foresight, Masuki is suspected of being poisoned in the court.
In order to clear the suspicion, Kuzumaru begins to challenge the many mysteries hidden in the capital of Heian.
Thr author wholehearted emergence of the Heian period mystery!
Seimei's childhood name in this story, Kuzumaru (葛丸) seems to be a reference to Kuzunoha.
The book is available on the Japanese Amazon website. I do not have access to this book myself. I was just looking for something to help take my mind off the anxiety and depression I am experiencing with the Onmyoji Mobile Game Plot.
Amazon.co.jp: 晴明変生 : 森谷 明子: 本