For someone who wants to say "sweet dreams" to someone in French, the phrase is "beaux rêves." It is often combined with "good night," in the phrase, "Bonne nuit et de beaux rêves."
There are certain phrases that are always good to know in various languages. "Where is the bathroom?" and "Hi, my name is..." quickly spring to mind. But there are other phrases that can also be fun to know. For students of French, learning how to say "sweet dreams" is a nice touch for late-night chats with another person who speaks the language. So when someone says, "Bonne nuit," or "good night," a student can respond with "Beaux rêves," or "sweet dreams."