What is the explanation about the Shia’ belief that Hasan and Husain will be the leaders of the youth in paradise?

Actually, this is a belief of Ahl-us-Sunnah as well, since it is reported from the Prophet (ﷺ) in hadeeths of Abu Sa‘eed al-Khudree and Hudhaifah ibn-ul-Yamaan (radiyallaahu anhumaa) in Jaami‘-ut-Tirmidhee (3768) as well as in other ahadeeth of many other companions.

Although, its roots have been subject to criticism, many reliable imaams of Hadeeth have graded it authentic including ad-Daaraqutnee, adh-Dhahabee and al-Albaanee.

And Allaah knows best.