Duryodhana points out the 'young blood' of the Pandava side. Abhimanyu (Saubhadro), the son of Krishna's sister Subhadra and Arjuna, was a warrior whose prowess rivaled his father's. The five sons of Draupadi (the Upapandavas) represented the continuity of the Pandava lineage. By labeling all of them Mahārathas (warriors capable of fighting 10,000 others), Duryodhana is signaling to Drona that there is no 'weak link' in the enemy's chain.