Keeping a BIG secret like this from Melody her whole like was a mistake and something the parents from Frozen would do but I actually kind of understand it and think it's more justified here than in Frozen. Anna should've known from the beginning and there was no reason for her not to know about Elsa's powers. With Melody, imagine having to tell a little girl that she has to stay away from the sea (Which she ADORES) and it's because of a psychopathic sea witch that almost killed her when she was a baby and was still planning to go after her. How would you explain that to, for example, a five-year-old? That would scar any kid for life! I took an Infant and Childhood Development this semester and things like this are bad for development. They needed to tell her, but I do think they needed to wait until she was the proper age, so she would be old enough to handle it and to understand the importance of the matter.

the thing about keeping a secret is that somehow it hurts more than knowing the truth. The truth would have hurt Melody for some time, but she would have trusted the right people. I have never come across a single individual who would have said, maybe it was for good that my parents lied to me, kept things from me about my life and real identity but i have across people who have overcome hurt implicated in childhood, with right amount of love, trust and guidance.

^ I think it depends on the person. I mean a seven-year-old could be fine watching scary horror movie where as a fifteen-year-old is wetting their pants and can't sleep for weeks. Besides, I think hearing about an evil witch that tried to kill you as a baby and is still after you is a lot more traumatic than anything most people in real life have had to go through with secrets. But I do see what you mean. It's not an easy situation.

From my perspective, there never would have been a good time to tell Melody anything, because Melody has inborn hostility towards her mother, towards her peers, and towards anything not related to her own interests. So Melody found out, and Ariel admitted the truth to her. Whatever.

From my perspective, the screenwriters ( suffering from writer's block) decided to mold Ariel into the shadow of King Triton...making her excessively worried and penalizing everybody for it. Meh. This totally rewrites Ariel's character traits, which have nothing to do with hatred and fear of the sea. She came from it, after all.

And as for Melody herself. Melody was a version of Elsa before Elsa existed. I found this out by watching Frozen and TLM 2 back to back. Much the same scenario...with Elsa blaming Anna for standing in her way and not leaving her alone...just as Melody blames Ariel for standing in her way, and not leaving her alone.

Neither are my favorite movies. And Melody reminds me too much of Elsa, so it's uncanny.

They have to telling the truth to her, but they have to wait until she was grown up. In her current age around 11-12 years old, they should tell her. Maybe the truth will hurt her but at least it was better to know it rather than she know the truth from other people while her closest peoples wouldn't tell it to her by themselves. It would make her losing her trust with them.

I mean, the story would definitely be different, but in universe, yes, she deserved to know.
I was thinking that the scene after the party would have been a good time to tell her, since by then, it was obvious Melody had some connection to the sea.