Perhaps not quite so straightforward in French. "Good night!" would be "Bonne nuit!". If someone wished you "Bonne nuit!", you could reply "Merci, pareillement" ("Thank you and the same to you"). "Bonne nuit" is only used just before going to bed. To say "Good night" when taking leave of someone during the evening, say "Bonsoir".
Sorry, I forgot the second bit. "Sweet dreams" would be "Fais de beaux r�ves" to someone you know well and "Faites de beaux r�ves" if you are saying it to more than one person. It is unlikely that you would say this to someone you do not know well but it would also be "Faites de beaux r�ves".