Does it send a notification to the parents that a nude photo was sent or viewed?

These articles discuss the feature you may be thinking about:

Child Safety - Apple

About communication safety in Messages - Apple Support

In the first one it says:

"In both cases, children are given the option to message someone they trust for help if they choose."

Interesting. I had thought that Apple had chosen to delay that feature. Good to know.

What is "it"? How would "it" know if a picture contained a naked person?

I suspect the answer is "no", no such notification is sent but, without more details, I'm unwilling to say that with certainty.

The answer is no, and is covered in the first article:

Apple does not get access to the messages, and no notifications are sent to the parent or anyone else.

tt2