Dear Mr Morsing,
I can’t speak for the law of FYR of Macedonia, but according to the law of Republic of Serbia and Statutes that regulate matters of citizenship, your clients child is the citizen of Serbia. Statute that regulates this matters (In Serbian its Zakon o državljanstvu from the year 2007) proclaims in article 9 that child born abroad acquires the citizenship of Republic of Serbia, if one of its parents is Serbian citizen. It is also necessary, according to this Statue, for that parent to apply to diplomatic or consular mission of Republic of Serbia, in order to begin procedure of registration of his (or hers) child in citizenship of Serbia. I advise you inquire further in the details of procedure in Serbian embassy to The Kingdom of Sweden. I am not quite sure where on our website have your clients found the information that you mention in your e-mail. I do believe that they misunderstood something in some of articles published on our website, since the law clearly speaks in their favor. I want to thank you for your e-mail, and i hope that you found my response to helpful.
With kindest regards and hope for future cooperation,