Question: Do the other children at Melody's birthday party know she's a mermaid? They must do if the snotty boys and girl joined Melody and all the other mermaids in the end.
Answer: Either possibility is true. They may not have known she was, which is why it was never said among them before, or they did and didn't say anything.
Question: If King Triton is the king of the sea, wouldn't he knew where Morgana was hiding? Do the humans know that Ariel and Melody are mermaids then if they do, do you think that those snotty kids would had told Melody?
Answer: King of the Sea does not mean psychic powers that allow him to be able to snuff out everyone's location. Only power he has is through the trident, which obviously cannot pinpoint location as he couldn't find Ariel when she was missing. It's likely the humans have no idea of the family's mermaid history, because yes, the snotty kids probably would have greatly teased Melody for it.
Question: Three questions: One, why does the Trident glow when Morgana is holding it? It never glows when Triton holds it, unless he's using it; Two, aren't Cloak and Dagger Stingrays? I'm asking because not only were they holding onto Eric's ankles but also, Sebastian was holding them after Eric was free. Wouldn't he be stung or something?; Three, why would Morgana make a "palace" out of ice? Could her tentacles hold on? I just think she would be slipping a lot.
Answer: 1.) It could be that the power-hungry Morgana is constantly keeping the trident in ready-to-fire mode, whereas Triton only summons its power when he needs it for something. 3.) She created a fortress out of ice because the only terrain around her was made of ice, and she wanted a structure she could use to fire down at the opposing forces while leaving them defenseless and unable to reach her. She didn't slip and fall because she arranged a tall platform in the center for her to stand on.
Question: When Melody escapes the castle she swims beneath that giant wall, through a hole and pulls out one of the metal bars that's there. Later on when she escapes the castle after arguing with Ariel, she happens to freely walk outside and pulls a boat and the sea's there. What happened to the hole?
Answer: She would only be going through that hole when she's swimming, but since she was walking on land there was no need for it. She could've gotten the boat and brought it to shore another way.
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.