Purity: Ghusl for Children and Disbelievers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 545:
Giving ghusl to a Muslim child is obligatory even if the child is of illegitimate birth. Giving ghusl, shrouding, and burying a disbeliever and his offspring is not obligatory; however, if the child of a disbeliever is mumayyiz and expresses belief in Islam, he is a believer. As for someone who was insane from childhood and reached the age of legal responsibility while he was insane, in the event that his father or his mother is Muslim, he must be given ghusl.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)