It's funny you bring up that Dutch mermaid tale, because I just heard a remarkably similar British version on Friday. The Mermaid's name is Asenora, and she is taken from the sea, made to live a human life, and becomes St. Senora. Sue Monk Kidd just came out with a book called _The Mermaid Chair_ that revolves around the legend. I don't know about the children's book you describe, but I was struck by similarities between the stories. I'm trying to track down the tale type.