They aren't played by the same person.
Who is the voice actress for the 'young' Nori-chan?
Teenage Sonozaki is played by YAMAMURA Hibiku, but child Sonozaki is played by 財前咲良 (probably read "ZAIZEN Sakura"). She's credited in episode 1 as "girl" (少女).
In episode 9, she's again credited, this time as "Sonozaki (child)" (園崎（幼少期）). Curiously, her name is ...