An effective way would be to build up the tension before this moment. Show Naruto going through a series of difficult events, like failed missions or arguments with his friends. Then, when the mask breaks, describe his sobs as gut - wrenching. 'Naruto's shoulders shook as he cried, the broken pieces of his mask at his feet. His cries were muffled at first, but then they grew louder, echoing through the empty alleyway.' This would make the readers feel the intensity of the moment.
In fanfic, Naruto's mask breaking and crying could be a turning point in the story. Maybe it's a sign that he's finally letting go of his past traumas. For example, if he's been haunted by the memories of his lonely childhood, this could be the moment where he confronts those emotions head - on. It also makes him more relatable to the readers as it shows his human side.
The author can describe her physical appearance while crying. For instance, they could write about the tears streaming down her face, her eyes becoming red and puffy, and her lips trembling. This gives a vivid visual of her crying.
You can show their physical reactions like trembling lips, streaming tears, and hiccups. Also, describe their inner feelings to make it more touching.
Describing the setting around him can enhance the mood. If he's crying in a desolate place, it can make the reader feel his loneliness more. Or if he's crying in a crowded room but still seems so alone, it creates a powerful contrast.
It could be out of jealousy. Sasuke might see Naruto as always hiding his true emotions and having this perfect image that others admire. By breaking the mask, he wants to expose Naruto's flaws and make himself feel superior. Or perhaps it's a way for Sasuke to assert his dominance over Naruto, to show that he can break through Naruto's defenses.
A writer can describe Finn's physical reactions. For example, they can write about his eyes welling up with tears that start to roll down his cheeks. Mentioning how his voice might crack when he tries to speak can also be effective. Another way is to show his body language, like his shoulders slumping as he cries.
Use vivid adjectives to describe the child's eyes, like 'wide - eyed' or 'sparkling'.
They can describe his physical state. For example, 'Baby Naruto's face was red as he wailed, his tiny fists clenched tightly.' This gives a vivid image of how he looks while crying.
Incorporate the five senses. Not just how things look, but how they smell (like damp earth), sound (a low moan of the wind), feel (the cold, clammy air), and even taste (the metallic tang in the air after a storm). This will fully immerse the reader in the saturnine setting.
Show the reactions of other characters. If the other ponies in the fanfic see Luna crying, their reactions can enhance the emotional atmosphere. For example, if they are sympathetic and try to comfort her, it shows that Luna is loved, but if they are indifferent, it makes her situation even more pitiful.