I'll be honest and say that today I was a little lazy, and just looked up this World of Wonder in my Dictionary of Imaginary Places. I haven't actually read L'Enfant de la Haute Mer (Jules Supervielle, 1931), though it sounds interesting. Instead, all I have is a short dictionary entry:
SEACHILD'S CITY, built on a floating island, somewhere in the north Atlantic, last seen at latitude 55 north and longitude 35 west. Visitors have great difficulty reaching the city becasue it disapaers as soon as a ship is seen upon the horizon. The entire city has only one inhabitant, the twelve-year-old daughter of a certain Charles Lievens de Steenvoorde, who disappeared on the high seas. The girl lives in this empty place, carrying out her everyday tasks in a normal manner. It is believed that there are many such floating cities built by drowned children whose bodies have never been recovered.
Completely enamored with the sea and abandoned places, I love the image of ghost children performing their chores all alone on little disappearing islands. I can only imagine that their hair is made of seaweed and that their skin is translucent.