This page of the FF7 Remake guide has a description of A Verified Hero side mission, available in Chapter 8 - Budding Bodyguard.
After completing the Kids on Patrol mission, you can return to the kids' camp. You unlock a new mini-game - Whack a Box.
The rules are simple. You have to destroy the boxes with points - this increases your score. When the time runs out, you will receive a proper reward. To complete the task, all you need to do is score 10,000 points, which is very simple. Remember, the bigger the box, the more HP it has.
It is best to stick to the ground floor - when you reach the fork, go left to find more boxes. Destroy the 1,500 points boxes with abilities - try attack as many crates as possible. Try to score 30k points - you will get an item that fills the limit gauge bar when using MP which is very useful.
Reward: Elixir. For 20,000 points - Crescent Moon Charm, for 30,000 points - Spectral Cogwheel.