Purity: Salat al‑Mayyit for Children Under Six and Stillborns
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 583:
Based on obligatory precaution, it is necessary to perform salat al‑mayyit for a child who has not completed six years of age but could understand prayers. If the child could not understand prayers, there is no problem in performing the prayer with the intention of rajaʾ [i.e. with the intention of performing it in the hope that it is desired by Allah]. There is no recommendation to perform the prayer for a stillborn child.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)