Lol, what was an interesting story.
Apparently Gaiman thought of the story when his son in anger stomped his feet and said, "I wish I didn't have a dad! I wish I had...I wish I had goldfish", and Gaiman being Gaiman thought, "well this is a brilliant idea. Of course one out to be able to trade father for goldfish. It seemed a very sensible thing to do!"

Whether the idea is brilliant or not is debatable but the book definitely was! 😍