"You are having breakfast."
Translation:Jullie zitten aan het ontbijt.
Only a beginner in Dutch myself, so I don't really know, but I suspect you need some article, if not "het" then at least "een". Also, it might be that "het ontbijt" is just the idiomatic way to refer to a breakfast, and that one wouldn't use "een ontbijt", at least not in the normal scenario. (I'd be happy if a Dutch speaker could confirm what I've just written.)