The Siren of Radhen Isle
The farmer Dhunus lived a pleasantly unremarkable life up until the day his wife and newborn daughter were laid to eternal rest in the childbed. On that day, knowing the divine mother Beatrix would be close as he buried them behind his farm, Dhunus called for her favor and pleaded with her to take away his pain. He begged the goddess for a new life but she did not so much as whisper in response, and so Dhunus finished burying the love that had been stolen from him and went back inside.
He soon found his own grief growing unbearable, and the farm itself ridden with too many memories. He knew if he stayed at the farm he would soon drown, and so he reluctantly packed a small bag of belongings and left for the port city—he planned to go as far as he could from the home that had filled with his sorrows.
Dhunus found the passage he was looking for within a day's arrival. The veteran captain of a merchant vessel had recently lost his entire crew, and now was looking for new men. Dhunus himself had served in the navy in his younger days, and eagerly approached the captain in search of work aboard the ship. The captain, a most daring and ambitious privateer who wore a cloak made of the finest sealskin and whose ship was more splendid than any ship Dhunus had ever been on, was thankfully desperate for a new crew.
"We go to seek the temptress who stole my crew," the captain told Dhunus. "A siren, a selkie, and each of my men fell to her allure. That crew was my everything! She stole the most precious thing I've ever had, and now I seek to steal the most precious thing that she's ever had in return."
Though it was not the noblest of ambitions, the captain's vengeance did promise adventure, and more importantly passage away from Dhunus' past. So Dhunus was hired as the vessel's boatswain, and off they sailed toward the siren.
It was many weeks before they came upon the isle on which the siren resided. She lived there solitarily, the captain had told Dhunus, and she could not leave. They dropped anchor when the isle was still a thin strip in the distance, as the captain feared his new crew falling to her charms if they moved too close. Thus, the captain would sneak onto the island by himself on one of the lifeboats and steal that which was most precious to her right behind her back.
The first night, the captain sailed off, and Dhunus was up and waiting for when he returned. The captain had stolen the richest of all the siren's belongings, a large, gleaming gemstone, and Dhunus asked him, "Did you steal it, the most precious thing she's ever had?" But as the captain brought the gem aboard, it turned to dust and they both knew then that it was not the most precious thing she'd ever had.
The captain stole thrice more from the siren, but each time the valuables turned to dust, and so finally Dhunus mused to him, "Perhaps the most precious thing she's ever had isn't an item at all." The captain was growing annoyed with the nights' failures by this point, but he thought long on Dhunus' comment and the next night sailed out again.
When he came back, he did not bring any items, and Dhunus asked as he helped him from the lifeboat: "Did you steal it, the most precious thing she's ever had?" The captain's mouth curled, a dry, callous kind of smile.
"Yes, I think so," he told Dhunus.
"No, you did not," a stern voice ushered out behind them. They turned to see the goddess Beatrix, anger burning through her ethereal form, and she fixed her harsh gaze on the captain. "I would kill you right now, if I could," she told him. "But alas, it is against the Law of gods and mortals. Instead, I bestow on you this curse: You have one more chance to steal the most precious thing the siren has ever had. If you are successful then you shall live eternally, but if you fail, your life will be forfeit and you shall perish."
She glanced briefly to Dhunus before she left, and it occurred to him what the most precious thing the siren had ever had must be: her life.
"Forget about the siren," he thus urged to the captain. "Let her go, and let us find adventure elsewhere." But the captain was angry as well now, and he would not be satisfied until he stole what was most precious to her. He promised Dhunus he would figure out what it was and steal it, and then his vengeance would be sated.
Dhunus was unnerved by the promise, and recalled the sinister manner of his smile. He would not divulge to the captain what he had realized, and could only hope that the captain would never figure it out for himself.
For a while, it seemed as if this would prove the case. They sailed to and fro, adventuring and plundering for ten months, while the captain puzzled over what the most precious thing the siren ever had could be. And Dhunus, though he had long buried his grief under the excitement and labor of sailing the open sea, now grew weighted with worry and unease. He did not understand what it was he felt—only that it was a deep discomfort that grew and grew the longer he sailed aboard the captain's ship.
Finally, the captain came up to him and said, "I've got it; I've figured it out."
"What is it?" Dhunus asked him. "What is the most precious thing she's ever had?"
"Her life." The captain grinned at his own revelation, and the discomfort inside Dhunus churned and roiled.
They sailed back to the siren's isle, and dropped anchor a ways away as they had before. The captain sailed out on the lifeboat, but before he left he grabbed Dhunus' arm.
"Come morning," he told Dhunus, "She will be dead, and you shall bring the ship in. Then we will gather all the jewels and valuable trinkets she keeps there for ourselves, and we will spend the morning on the isle and drink to our triumph." He smiled at Dhunus, and though it was meant to be assuring, Dhunus' insides curled. "A new life, that was what you told me when we met, yes? And here it shall be, a new life, come tomorrow!"
The captain set out, and Dhunus paced back and forth all through the night, wondering if he had succeeded. Dhunus even prayed to Beatrix despite his bitterness at her not answering his own call before. Save her, he pleaded, do not let his vengeance steal her life! The goddess didn't answer this time either.
Come morning, he steered the ship toward the isle. The crew dropped anchor, and Dhunus bid them wait on the vessel while he went ashore to find out what had happened. He made his way to the beach, and then to the cave where the captain had told him the siren resided. All was silent save for the soft wash of wind and waves and birds all around, and it unnerved Dhunus as well that the captain had not been there to greet him on the beach.
He entered the cave, the stench of blood hitting him even before his eyes adjusted to the dim light. The siren drew his attention first, beautiful and unmoving, and then the pile of dust and a sword to her side. Dhunus looked around but his captain's body was nowhere to be found, and then he looked once more at the siren, at her bloodied dress and red smears across her arms and legs, and also at the dust and dagger. He could surmise what had happened and though it was tragic, he was also strangely relieved at the captain's death.
He surveyed the scene for a moment longer, his eye catching on a large basket in the corner of the cave. Stepping over, he knelt down and peeled back the wrapped blankets inside. A small breath escaped him, perhaps his grief giving way just a breadth to something else. He considered the siren one last time, her eyes open and mouth parted as if to let her soul escape.
And then he stole the most precious thing she'd ever had, and returned to his farm to raise his newborn daughter.
Considering the roots of many modern fairytales, like The Brothers Grimm, this is fairly tame. It helps explain the will of the Gods/esses in your world while also illustrating how precious a child's life is. It also makes sense if this was a story that might have been used to paint the Cravvik people in a certain light. I think it does fall into the realm of children's stories for your world with all that in mind. :)