By Hans Christian Andersen
The Andersen original, not the Disney one: the mermaid gives up everything for a prince who marries someone else.
Not the cartoon. The youngest daughter of the sea king longs for the world above the water, and when she saves a prince from drowning she falls hard in love with him. To live on land she trades her voice to the sea witch for a pair of legs. The witch warns her that every step will feel like walking on knives, and that there is no changing back. With no voice, she can never tell the prince who she is or what she gave up. He grows fond of her, but he marries someone else. On his wedding night her sisters bring her a knife: kill the prince, they say, and she can return to the sea as a mermaid. She won't do it. She lets herself go instead, and turns to sea foam at sunrise, carried off as a spirit of the air.
If your child already knows the singing movie by heart, the real Andersen story is a genuine step up, in the language and in the feeling both. It doesn't hand you an easy ending. It's about loving someone who doesn't love you back the same way, and choosing not to hurt him even when it would save your own life. I'd read this one sitting right next to them, and go in knowing it parts ways completely with the happy ending they're probably expecting.