Actually, it is "Kaffee und Kuchen" or "Kaffeetrinken". It is even used when you don't drink coffee or eat cake. In Germany, we ate as afternoon snack a piece of cake and drank coffee but now it is more like a proverb and stands for eating something in the afternoon and making it "gemütlich".

Pausenbrot, Imbiss, Naschis, eine Kleinigkeit (essen), etwas kleines zwischendurch essen you can use these expressions and just add the time you're having the snack, but normally there's no reason to specify when you're having a snack.

Vesper in Switzerland and South Germany. Jause in Austria.

It depends on what you actually eat. "Kaffee und Kuchen" means you have coffee and a piece of cake, maybe together with others. Literally, "afternoon snack" would be "Nachmittagsimbiss", "Nachmittagssnack", "Snack am Nachmittag" or something like that. Linguee has a wide range of translations, including the generic "Zwischenmahlzeit": http://www.linguee.de/deutsch-englisch/search?source=auto&query=afternoon+snack