With his first wife the actress Lydia Brincken Sirk fathered one son, Klaus Detlef Sierck (1925–1944), born on 30 March 1925 in Berlin-Charlottenburg, Germany. His ex-wife joined the Nazi party and because of Sirk's remarriage to a Jewish woman was able to legally bar him from seeing their son, who became one of the leading child actors of Nazi Germany, known for "Die Saat geht auf" (1935), "Streit um den Knaben Jo" (1937) and "Kopf hoch, Johannes!" (1941). He died as a soldier of the Panzer-Grenadier-Division Großdeutschland on 22 May 1944 near Novoaleksandrovka, Kirovograd Oblast, Ukrainian SSR, USSR (now Novooleksandrivka, Kirovohrad Oblast, Ukraine).