ひなって英語でなんて言うの？

Chick is the most common usage but is also a slang word to describe a human female. Nestling is a bird that has yet to leave its nest. Fledgling is a bird that is yet to learn to fly.

*Chick-Birds lay eggs and when the eggs have matured they hatch, and there we have chicks. Birds lay eggs in a nest.

A nestling is a young bird that has yet to leave its nest, so it may have just hatched or could be fully fledged yet not ready to leave the protection of the nest.
A fledgling is a young bird whose feathers and plummage are in the development stages. A hatchling is a young chick recently broken out of its egg.

There are different terms to be used for a baby bird, you can just say 'baby bird', a hatching is a bird that has just been born (hatched) from the egg
A Nestling is a baby bird that has not yet left the nest yet
A fledgling is a baby bird that can not fly yet but has left the nest

A chick is any baby bird.

A hatchling is a bird that has just emerged from an egg. It is a very young bird.

A fledgling is a bird that has grown feathers and is about to fly for the first time.

Chick is the name given to a baby chicken.

is also a slang word for a woman
example use "hot chick"

The most common word for a baby bird is a "chick" but you can also call it a "baby bird" or "Nestling.
These are all words to describe a small bird that lives in a nest and cannot yet fly.