How do you say good night princess in welsh?

There isn't a direct translation for "good night princess" in Welsh. However, you could say:

Nos da, frenhines (Nos da, frenhines)

This translates to "Good night, queen".

You can also use a more informal greeting, such as:

Nos da, cariad (Nos da, cariad) which means "Good night, darling."

You could also say "Nos da, fy frenhines" (Nos da, fy frenhines) which means "Good night, my queen".

The choice will depend on your relationship with the person you are speaking to.