It is not in an episode, in the first episode of Dragonball Z it shows Guko with his son as a child.
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Goku Defeats Baby In Episode 39
Baby Trunks comes out in the episode where Goku and the others start finding the Androids more specifically Episode 72
Goku turns SS4 in Episode #35 gt.
The episode in which Chi Chi and Goku fight is episode 137. It's also the episode in which Chi Chi becomes Goku's fiancee.
Goku turn into kid Goku in episode 1 or Dragonball GT because Emperor Pilof accidentally wished for Goku to be a kid again.