Formal:
* Goeienaand, my liefie. (This is the most literal translation and is suitable for any situation)
* Goeienaand, my skat. (This means "good night, my treasure" and is a more affectionate way to say it)
Informal:
* Slaap lekker, my liefie. (This means "sleep well, my love" and is very common amongst couples)
* Nagi, my liefie. (This is a shortened, informal version of "good night" and is more common among friends and family)
Romantic:
* Goeienaand, my hartjie. (This means "good night, my little heart" and is very romantic)
Which one you choose depends on the context and your relationship with the person you are saying it to.