How do you say goodnight to both of you?

It's appropriate to say 'buenas tardes' any time after 12:00 noon. It's O.K. to fudge a few minutes before noon. You can continue to say it right up to near dusk, or the dinner hour. The dinner hour usually is the watershed for beginning to say 'buenas noches.' If you meet someone at 6:30 p.m. in the summertime, and they say 'buenas noches' while the sun's still shining, it means they've already had dinner, and this is 'night' for them.