On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough of the Battle Born side quest in the Hordafylke region.
Reward for completing the mission: 1200 XP.
In the place marked on the image above, you will meet a group of Viking children playing. Join them and follow the group to the battle area.
Eliminate all the targets in the makeshift village and talk to the leader child, Kaija, to complete this quest.