Excerpt from 'Story of the Holy Ka'aba And its People':
Imam Hasan’s (as) 3 sons, Qasim, Abdullah and Hasan-e-Muthanna, were in Karbala’ and took part in the Jihad with their uncle Imam Husayn (as).
Excerpt from 'Maqtal al-Husayn':
...His first name was ‘Abdullah al-Akbar [‘Abdullah senior] and his mother was an 'umm walad' named Ramla. He fought till he was killed.
After the latter, his full-blooded brother, al-Qasim, came out. He was a lad who had not yet come of age. When al-Husayn (‘a), looked at him, he hugged him and wept.
Then he permitted him to fight, so he came out with a face looking like a full moon bearing a sword and wearing a shirt and a mantle. On his feet he wore sandals. He had to fight on foot.