Hadith.488 - He was asked : "When does the funeral prayer become obligatory for a child?"
Imam (as) replied: "When the child understands the concept of prayer and is six years old."
Al-Kāfi #790“Abu ‘Abd Allah (a.s.), was asked about Salah (prayer) of a child. He (the Imam) said, ‘There is Salah (prayer) for him when he is able to...