Karn had four sons - Vrishasen, Chitrasen, Satyasen, Sushen.Arjun killed Vrishasen when Karn was commander of Kauravas. This means that Vrishasen was killed before Karn.Nakul killed other three sons of Karn after Karn had been killed. This happened when Shalya was the commander of Kauravas.If we see the order in which Karn and his sons were killed, then we find the following sequence: -1. Arjun killed Vrishasen.2. Arjun killed Karn.3. Nakul killed Chitrasen.4. Nakul killed Satyasen.5. Nakul killed Sushen.So sad to see that even youngesters like Karn's sons and Draupadi's sons fought in that war. It was their age to enjoy life but they fought and were killed in the war.