I have read two books recently that retell (in different ways) Matthew Arnold's The Forsaken Merman. Firstly, Liz Kessler's The Tail of Emily Windsnap. Emily lives on a boat in a seaside town with her mother. Despite them living so close to the sea, her mother has never allowed Emily to learn to swim. During a swimming lesson, Emily finds out why: as soon as she hits the water, her legs fuse to a tail, and she becomes a mermaid! Believing that her mother is terrified of water, she doesn't tell her of her discovery; instead she puts her new tail to the test, swimming further and making a new friend. However, she discovers that merpeople are forbidden to have contact with humans, and that her mother is being betrayed dreadfully by someone who has a reason to keep it that way... A great, fun read.
I'd recommend both books for 8+, Forsaken particularly for children who enjoy stories but find reading quite daunting. And the title of this post? The jaunty song from Disney's The Little Mermaid, of course!