Continuity mistake: When Melody is in her room getting ready for her party, at one shot her hair is down. In the next shot of her, it's in a ponytail, and in the next it's down again.
Continuity mistake: When Morgana traps melody in the cavern, Flounder is seen in his younger size. The next time Flounder is seen he is back in his adult size.
Continuity mistake: When Morgana sinks Prince Eric's ship, he asks where Melody is and draws his sword. When Cloak and Dagger (the manta rays) pull him under the water, he no longer has his sword in his hand.
Continuity mistake: When Melody is singing for a moment, as she rises out of the sea on that huge whale, her green ribbon is missing.
Answer: Melody was only a mermaid for a short amount of time and they never saw her that way. By the end of the movie when she used the Trident to take the walls down, she was in normal human form, and so was her mother. The fact that other mermaids were around might have been a shock, but Melody could've just explained that off-screen. Either way, she was still kind enough to let them join.