I explain it a hundred times with the "not in copulative sentences" disclaimer, and this time I left it out. :´)
So: in copulative sentences the question words "who" and "what" can be plural, too. But if you start using a different verb than the plain "to be", plural forms are not used with "who" and "what".
Shouldn't it be 'Who walk', because 'Kik setálnak' is plural.
No. English does not have separate singular "who?" and plural "whos?" like Hungarian ki / kik. "who?" in such sentences is always singular in English -- thus we say "Who walks?", even if we know or suspect that the answer will be plural.