This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Goten's Growth side quest.
Time frame: From Majin Buu Reborn - Episode 1: New Encounters to Episode 1: Good News
Reward: 190,593 EXP, 10x D Medal, Super Sacred Text of Martial Arts, Card 069
Where to start: Southeast Mountains Area, as Gohan.
This mission begins with a conversation with your little brother. Mom will send you fishing. Your task is to catch 3 blue fish.
While fishing, you can catch many other fish. You can sell them later to get more money.
Another part of this task is to spar with your younger brother. This fight can be challenging. Despite the young age, Goten has a high level and a variety of attacks.
After defeating your brother, you must return home to finish the mission.