Sebastian: That's it. I'm booking meself on a cruise.
Melody: Do I have to do this? Everybody thinks I'm weird.
Ariel: We have to keep Melody away from her. Until Morgana is found Melody can't go in the sea.
Flounder: But Ariel.
Ariel: I'm sorry Flounder, but if Morgana is anything like Ursula, she will never give up. This is the only way. Melody can't know about merpeople or Atlantica or even about you, Daddy.
King Triton: Very well, Ariel, you're right.
Melody: What a totally cool feeling.
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.