With the English word 'sexy' having infiltrated the modern Italian vocabulary, the most popular Italian translation of 'goodnight sexy' is buonanotte sexy.
Sexy is sexy
Italians don't have an exact word that directly replicates the meaning of 'sexy' so, to deal with this, modern Italians have simply adopted the English word into their own language.
In fact, the loanword 'sexy' is so commonly used that it is often preferred over equivalent Italian words like sensuale (meaning sensual).
Goodnight in Italian
To wish someone 'goodnight', Italians use the expression buonanotte with buona being the female adjective form of 'good' and notte being the translation of the word 'night'.
To simply wish someone a good evening, Italians would use buona sera.
Other sexy Italian words
One colloquial term used to describe someone sexy is bono (male) and bona (female) as in the example quanto sei bona! or 'how sexy are you!'
This is a phrase which you may hear used as a chat up line in various parts of Italy - with varying degrees of success.