On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough for the Hide and Hunt side mission.
Rewards: 1,200 XP.
In the place marked in the picture above, you will meet a group of children. When you talk to them, Sigg will tell you that they are playing hide-and-hunt – your task is to hide so that the boy can't find you.
Jump on the roof of the hut and crouch – that the boy won't be able to find you. Wait here until the rest of the participants are caught. Sigg won't notice you even when he walks past the hut. When the boy starts shouting that he gives up, come out of the hiding spot and talk to him to finish this quest.