The hadith reported by Ahmad, At-Tirmithi, Ibn Maajah and An-Nasaa’i, it was narrated by Abu Sa’eed Al-Khudri may Allaah be pleased with him who said: “The Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) said: ''Al-Hasan and Al-Husayn are the masters of the youth of Paradise.” Shaykh Al-Albaani may Allaah have mercy upon him classified it as Saheeh [sound].
The hadith doesnt negate other people from holding the title "masters of youth". The hadith doesnt rule out third son of Ali and fatima, muhsin. Provided there is consensus (ijma) which says muhsin will be in paradise.
Imaam al-Nawawi said: the reliable Muslim scholars agreed that any Muslim child who dies will be among the people of Paradise, because he was not responsible (i.e., had not yet reached the age of account). (Sharh Muslim, 16/207).
So the question is do we have other masters of youth in paradise?