This page explains how to start and complete the Playtime with Cottla side quest.
How to Start the Quest
Talk to Cottla between 11AM and 4PM in Kakariko Village, and agree to play, to start the quest.
How to Complete the Quest
If you wait too long, Cottla will have to go home for dinner. If this happens, you will need to talk to her again between 11AM and 5PM the following day to proceed with the quest.
You can choose to either play Hide and Seek, or Tag. For Hide and Seek, there are various spots in town where Cottla may hide. For Tag, you and Cottla will start in different parts of the village, and you have to catch up with her and talk to her.
Cottla will reward you with Rock Salt or Amber. You can talk to her again to repeat the quest.