"Tu es mignon/mignonne" (depending on whether you're talking to a boy or a girl).
For a girl, you would say "belle" or "jolie", and for a boy, you would say "beau". That is why the name Beau was very popular for boys as it meant "cute" and "handsome".
This is how you say "You are Cute" in French. "Vous êtes mignon."