The entire emotional ordeal (and dismal failure) for Melody at her own twelfth birthday party. She's already considered strange by her peers, but her parents' encouragement before the party eases her worries and helps her to believe that everything will be alright. Unfortunately, disaster strikes (through no fault of her own), she's utterly humiliated, everyone laughs at her, and she flees the ballroom in tears. Seeing her crying alone in her bedroom afterwards is difficult to watch.
Ariel trying so hard to console Melody, but to no avail.
Melody:(crying) What's wrong with me?
Ariel:(sadly) Nothing's wrong with you—
Melody: Mom, I'm the princess of disaster!
Following this is Melody and Ariel's argument. Melody finds her locket (that Ariel gave back to Triton years earlier) during her adventures in the sea, much to Ariel's shock. Melody asks her mother about the locket, but Ariel avoids her questions and instead scolds her for going out to the sea. Melody sobs and tells Ariel she won't know anything about the sea since she's never even been in it (unaware that ironically, she has been in the sea and was even born there) before she runs out of her room with her locket. Melody just had a disastrous birthday party, and now she's being scolded by her mother for her love for the sea, not understanding why she hates it (in her thoughts).
Morgana: (to the sky after besting Triton) See that, ma!? WHO'S YOUR FAVORITE NOW!?
At the beginning, when Ariel decides it's best for Melody's safety to not know anything about the sea. Possibly more tear-jerking is that keeping Melody ignorant is exactly what leads her into trouble. Imagine that as a parent: you do what you think is best for your child, and your actions turn out to be more harmful than helpful.
Also, poor Triton, as this meant that he was going to have to avoid his own daughter and granddaughter for the latter's safety.
Ariel finding out Melody ran away, and her realizing it's her fault for doing so.
The image of Melody being changed back into a human while in the cave and nearly drowning as a result.
Ariel's singing part in "For a Moment". Unlike Melody's part where she is absolutely loving being a mermaid, Ariel is completely out of her mind with worry for her missing daughter; at one point singing in a painful note on why Melody had to leave. It's clear that Ariel is loathing her decision for not being honest with Melody about her heritage.
