On this page of the Assassin's Creed Valhalla guide, you will find a detailed walkthrough description of the side quest Alisa in Wunderlandscire.
Rewards: 1,200 XP.
In the place marked in the image above, you will find the entrance to a cave from which a child's voice can be heard.
Inside you will find a girl. Throw her on your back and carry her out of the cave. Once you are outside, the mission will end.