The naming of Hazrat Imam Hussain radiallahu taala anhu and Hazrat Imam Hassan radiallahu taala anhu

When Hazrat Imam Hussain radiallahu taala anhu and Hazrat Imam Hassan radiallahu taala anhu were born on different occasions their father wanted to give them other names, but the Holy Prophet Muhammed Salalaahu Alaihi wasallam specifically named them from these names, these names were specially saved and kept for them .

The names of “al-Hasan“ and “al-Husayn“ were not known before that; i.e. in the state of ignorance (Jahilyah). In fact their grandfather Prophet Muhammad sallallahualayhi wa sallam was the first one who named with them(who named somebody with these names).
Ibn al-Athir said in “Asad al-Ghabah”

Leaders of the youth of paradise
After they were born the Holy Prophet Muhammed Salalaahu Alaihi wasallam did not give them a basharat that they will be heaven , but he gave a Basaharat that they will be leaders of the youth of paradise.

The Prophet – blessings and peace be upon him – declared: “these two sons of mine are the leaders of the youth in Paradise.“
[Sunan al-Tirmidhi, Kitab al-Manaqib, Manaqib Abi Muhammad al-Hasan, Hadith no. 3793, Vol. 5, Page 426].

Narrated: Ibn Umar
The Messenger of Allah (saw) said: ‘Hasan and Husain will be the leaders of the youth of Paradise, and their father(Syadinna Maula Ali radiallahu taala anhu) is better than them.
Sunan Ibn Majah , Chapter 1 , Book of Sunnah Hadith no: 118

The Holy Prophet Muhammed Salalaahu Alaihi wassallam had to choose between his son and grandson , and he chose his grandson.

It is stated that once, the Holy Prophet – blessings and peace be upon him – placed Imam Hussain on his right thigh and and his own son, Sayyiduna Ibrahim on his left thigh. Jibra’il appeared to him and said, “Allah will not keep them both with the Prophet, choose one of them.” The Holy Prophet – blessings and peace be upon him – could not bear being separated from Imam Hussain and three days later, Sayyiduna Ibrahim passed on to the Divine Lord. After this incident, whenever Imam Hussain came in front of him , he used to kiss him and say, “Welcome to the one for whom I have sacrificed my own son
[Tarikh al-Baghdad, Vol. 2, Page 200].

The reward for those who are their friends .

Hadith 1
He has also declared that, “They are my sons and the sons of my daughter. O Allah! I have kept them in my friendship; You also keep them in Your friendship and keep him as Your friend who keeps them as his friends.
[Sunan al-Tirmidhi, Kitab al-Manaqib, Hadith no. 3794, Vol. 5, Page 427].

Hadith 2
He has also stated that, “these two are the swords of the heavens. “He has also stated that, “Hussain is from me and I am from Hussain. May Allah keep him as a friend who keeps Hussain as his friend. Hussain is a grandchild from among grandchildren.
[Sunan al-Tirmidhi, Hadith no. 3800, Vol. 5, Page 429].

Hadith 3
The Holy Prophet – blessings and peace be upon him – has also stated that, “Their friends are my friends, their enemies are my enemies.
[Sunan ibn Maja, kitab al-Sunna, Fadl al Hasan wa al Husain, Hadith no. 143, Vol. 1, Page 96].