Unarmed Child

Karna was a suitor for Draupadi at her Swayamvara, attempting to win her for Duryodhana. Unlike most other contenders, he was easily able to wield and string the bow, but Draupadi refused to allow him to take part, rejecting him for being a "suta-putra".The entire kings and the assembled people insulted Karna for his low birth; only Duryodhana supported and argued for Karna, saying 

1. Great Sages, 
2. Philosophers, and 
3. Warriors have no source. 

They are made great, not born great. Having escaped Varnavat, the Pandavas were also present in the swayamvara, disguised as Brahmins. Following the failure of the other princes, Arjuna stepped into the ring and successfully hit the target, winning Draupadi's hand. The assembled kings in the Swayamvara argued that a Brahmin was ineligible and they took their weapons and attacked the disguised Arjuna. Arjuna fought with all the assembled kings and defeated them with ease.

Karna entered the battle to protect his friend Duryodhana; a terrible battle was fought between Karna and Arjuna, but Arjuna recognized Karna and the two Maharathis fought each other. The battle was so intense that sky was filled with arrows and visibility was lost in the cloud of arrows. With both archers not gaining upper hand, they turned to use celestial weapons. With the battle deadlocked,

News came to Karna that his nine-year-old son Sudama (not to be confused with the more well-known Sudama, Krishna's childhood friend) was dying. Karna praised the skill of the Brahmin and compared it with the skill of Drona or Bhishma. Not being interested in committing Brahmahatya, Karna withdrew from the fight and rushed to his son. It was then revealed that Sudama was hit by an arrow sent by the Brahmin warrior when fighting earlier with the other kings. The bystander Sudama died.

When Arjuna's identity was later revealed, Karna's feelings of hostile rivalry with him further intensified and he swore to kill Arjuna and his family.

Later Karna avenges this by making abimanyu the child of arjuna unarmed from behind