This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for Daddy's Little Girl side quest.
Time frame: From Majin Buu Reborn - Episode 1: New Encounters to Episode 1: Good News
Reward: 198,495 EXP, 3x D Medal, Golden Compass
Where to start: Southeast Islands Area, as Gohan.
To start this mission, you must get on the bridge where Krillin and his family are standing.
Your task is to catch a blue fish and retrieve the child's item. This will complete the mission.