What do you call a secret writing name?

Answer 1

Use a code or invisible ink.

There are many different codes a simple one is to use numbers 1-26 that represent the letters of the alphabet.

Lemon juice can be used as an invisible ink. Write using lemon juice and to read the writing heat the paper (with an iron ) and the writing will appear

Answer 2

Not a clue. So any help for my six year olds homework kindly received.

How about pen-name or pseudonym? I feel bound to say that a school should not be setting such questions for six-year-old kids ... It simply tests the ability of the parents to find the answers. For most six-year-olds it is quite an achievement to understand what is meant by writing a book. They cannot be expected to know that some authors want to keep their identities secret.