Be they cute little girls, or perverted old men, if they want to play on your sympathy, they will turn eyes huge and shining with unshed tears upon you.
Generally, the younger the character doing it is, the more effective.
The reason this trope exists is because in real life children's eyeballs will have grown to their full adult size by age 3, making them look disproportionately big at that time.
Puppy-Dog Eyes has been suggested to play 1 roles. Click below to see other actors suggested for each role, and vote for who you think would play the role best.