The quest is unlocked after completing the Each Step Like Thunder scenario.
"The Zora children are begging Sidon to give them swim lessons. They all wanna be super-fast swimmers just like Mipha! Help Sidon get some supplies to ensure the training goes smoothly."
"Sidon spent hours teaching the children how to swim upstream. He could not quite hide his pained expression when, without a word of thanks, they swam off to show Mipha their new skill."
- Sidon: Bonus Combo