On this page of the Assassin's Creed Valhalla guide, you'll find a detailed walkthrough for the Warmth of Winter side quest.
Reward for completing the mission: 1200 XP.
In the place marked in the picture above, you will meet a child trading goat figurines for the Yule holiday. When you speak with him, you will learn that the goats are stored in a safe place. Buy one figurine for 100 silver, then run after the girl.
Your child will lead you to her sister's grave and tell you that they used to make goats together, but last winter her sister died. The girl promised her sister that she would continue their work. After a short conversation, the quest will end.