Ok, this is kinda random...
But say Melody hadn't run away when she found the locket. Ariel and Eric were going to explain everything to her, right? But if your mother came up to you and said 'Hey, I used to be a mermaid' would you believe her? Yeah, Melody wanted to be a mermaid but that doesn't mean she'd believe Ariel. She'd probably think she was patronising her or something.
Plus, wouldn't she have freaked out when Ariel became a mermaid again? I mean, no offence to mothers or anything, but if my mother came swimming up to me with only a bra on I think I would freak out. Just a thought.