What is the name of fish child?

There is no common, universally accepted term for a "fish child". This phrase is not a recognized biological or anthropological concept.

It's possible you might be thinking of:

* Merfolk or Mermaids: These are mythical creatures from folklore, often depicted as human-like with fish tails.

* Fish people: This term is sometimes used in fantasy literature or role-playing games to describe people with fish-like characteristics.

If you have another specific concept in mind, please provide more context so I can help you further.