My recommendation would be to avoid using `shop=kiosk` . It's a tag which has been documented since the early days of wiki tag docs. I had [some persuading to do][1], to convince people that `shop=newsagent` was a good tag, because it's selling the same things as far as products go, so was sort of a duplicate. As we know, the word "kiosk" describes the building size. For this reason it never really should've been in the 'shop' key (Not that I'm blaming anyone. It was a very early tag a as I say)
Now that shop=newsagent has bedded in a bit, if I could bring myself to start an argument on the matter, I would probably suggest that `shop=kiosk` should be phased out. This doesn't mean we stop mapping "kiosks". We just use a tag like building=kiosk instead ([a few people starting to use this][2])
So in answer to your question I would do: `amenity=cafe, building=kiosk` ...and you could throw in `seating=no` for good measure.
You might say, because it's only a little kiosk, it's not really a cafe. Debatable I'm sure, but I tend to think in terms of "top level tags" to give a basic classification to a thing. In this case the most helpful basic classification would be cafe I reckon.
[1]: http://wiki.openstreetmap.org/wiki/Talk:Proposed_features/Newsagent#Not_needed
[2]: http://taginfo.openstreetmap.org/tags/building=kiosk
