Andrew: My God, are we gonna be like our parents? Claire: [teary] Not me...ever. Allison: It's unavoidable, it just happens. Claire: What happens? Allison: When you grow up, your heart dies... Bender: Who cares. Allison: I care.
The heartwarming part is that the five kids are now aware of what could happen to them, and by movie's end all five seem willing to defy that fate.
Plus the reassuring nod Bender gives Claire when she says she won't ever be like her parents was surprisingly touching.
Unless it was actually a sarcastic nod that meant "Yeah, right."
When Andrew sees Allison after her makeover, he stares shocked at her and says, "You're just so different. I can see your face."
Allison: *nervous* Is that good or bad?
Andrew: *smiling* It's good. *she smiles too*
Before then, she looks to Brian and he smiles, she smiles back and silently thanks him. It's very sweet.
This line of dialogue that takes place after Allison's makeover, while absent from the final cut of the film but still present in the trailer, certainly counts:
Allison: I don't want to be alone anymore.
Andy: You don't have to be.
Claire assuring Brian that it's perfectly okay that he's still a virgin. After Bender makes fun of him for it, it's nice to see Claire offer him some sincere reassurance. The way they smile at each other is adorable.
Andy and Allison, of all people, defending Claire when Bender mocks her 'talent'.
Bender's comment when Andy spills his guts about his dad ("I think your old man and my old man should get together and go bowling...") is an oddly touching show of sympathy, considering how antagonising Bender usually is towards the others.
Basically any time the kids pull together and lie to Vernon about what's been going on while he's out of the room, especially at the start of the movie. None of them like each other, and could easily have ratted one another (mostly Bender) out, but instead they cover for each other.