"Fais de beaux rêves" (pronounced "fay duh boh rev")
This literally translates to "make beautiful dreams".
You can also say:
* "Bonne nuit" (pronounced "bon nwee") which means "good night"
* "Dors bien" (pronounced "dor bee-an") which means "sleep well"
"Fais de beaux rêves" is the most direct translation of "sweet dreams", though.