Ok, the problem of "was Childe a menace or a timid boy as a kid" is actually solved through his 5th character story. Thanks to @letpaimonbitepeople for pointing it out. The EN translation team was at it again.
在父亲眼中，本就不安分的三子变得顽劣异常，为平和的海屑镇带来了诸多无妄的纷争
Official translation: "In the eyes of his father, that third son that he had so worried about had changed for the worse, bringing much uncalled-for havoc to the seaside village of Morepesok."
A more literal translation:
"In his father’s eyes, his third son, already restless by nature, had become exceptionally unruly, bringing many unnecessary conflicts to the peaceful town of Morepesok."
The first lines of the same story still exist, of course:
回歸家鄉之後，少年變了一番模樣。 他不再膽怯、猶豫，而是變得輕狂、自信。
"After returning to his hometown, the boy changed completely. He was no longer timid and hesitant, but became arrogant and confident."
It could mean that before he was somewhat more restrained (a hyperactive but anxious kid), and after the Abyss even that restraint was gone. Or maybe he was a quiet menace who turned into a more socially outgoing menace.
Anyway, the contradiction solved. There was never any huge change of character in him, just the sharpening of natural tendencies.
