Dragon Ball Daima’s announcement in 2023 thrilled fans at the prospect of more canonical adventures in Akira Toriyama’s world, but a burning fan question about why Goku and other characters shrunk is already answered. As shown in episode #1, after Demon King Dabura was defeated, effectively making Gomah, the new King of the Demon Realm. Gomah then uses Shenron to turn Goku and his fellow warriors back into children in Dragon Ball Daima. This aims to weaken threats to Gomah’s rule in Dragon Ball Daima, with Goku, Vegeta, and others’ child states being significantly weakened as a result.
https://screenrant.com/why-goku-shrunk-dragon-ball-daima-explained/