Translation:A girl and a boy are singing together.
They mean the same thing as flicka and pojke pretty much. Flicka and pojke normally refer nowadays to younger children whereas tjej/kille are young but also older, making kille similar to English guy. It is a known problem that you are being tested on them without them being introduced, so if that is the case you should report it.
Yes. Elsewhere "killar och tjejer" is translated as "guys and gals," and so "gal" for "tjeje" must somehow be overlooked here. However, the comments in this discussion that the terms generally refer to teenagers, as well as more colloquial than "pojke" and "flicka," are helpful.