There is a folk tale out there somewhere with the storyline going along the same principle. A king is delivered a prophecy that his newborn son will be the one who kills him. Instead of fearing him, the king raises his son with love. In the end, his son gives him a mercy killing on his deathbed so he doesn't have to suffer.
I feel this will be one of those stories, especially now that they're married. We don't know if elves are a long-lived race in this universe so she will likely pass away from old age and Kakyu will ask his son, as the only person powerful enough to do so, to let him move on with her. He'll definitely be an adult by then.
Either that or Ars becomes unhappy to the point it breaks the promise.
It’s cute but I’m not going to follow the updates because I don’t want to cry if the baby does grow up to kill his father