Humans meet primitive aliens with six fingers, they turn out to be more intelligent than humans, there's a tropical kind of planet, and something to do with a young alien.
It's been 6 years since I've read this short story and I'd like to know what it was.
This is likely to be "Turning Point" by Poul Anderson. See this excellent previous answer providing details to confirm.
The story does indeed involve an alien planet with "primitive" inhabitants (called Jorillians) who start advancing in technology at an incredible pace after even brief exposure to human technology and ideas. The most prominent alien character is described this way:
A girl was emerging from the woods. She was about the equivalent of a Terrestrial five-year-old, I estimated. Less than a meter tall (the Jorillians average more short and slender than we), she had the big head of her species to make her look still more elfin. Long blondish hair, round ears, delicate features that were quite humanoid except for the high forehead and huge violet eyes added to the charm. Her brown-skinned body was clad only in a white loincloth. One four-fingered hand waved cheerily at us.
And again as:
The little girl (damn it, I will call her that)
