Quick answer: A Memory Flower about 66 m from the Pangzhen Slums bell starts it. A second flower about 85 m from the Pond of Fading Song bell finishes it, once you kill the monster guarding it.
First Memory Flower: Pangzhen Slums
The first flower is in Pangzhen, roughly 66 m from the Pangzhen Slums bell. Interacting with it starts A Mother’s Heart, and the tale’s objective changes to “Discover the whereabouts of the woman’s son”. This flower is one of several things worth picking up in the area. The full set is covered in the Pangzhen collectibles list.
Second Memory Flower: Pond of Fading Song
The other flower is roughly 85 m away from the bell at Pond of Fading Song. A monster is stationed there; defeat it before you reach the flower.
Once the guard is dead, interact with the flower to finish A Mother’s Heart.
A Mother’s Heart reward
Finishing the tale adds the medicine Merciful Dew, whose card reads “Restores 60% HP over 10 seconds.”, along with a memory item.
