What is the ruling on the funeral prayer for a deceased child, and when is a child considered an adult, and is he prayed for forgiveness like an adult, and is it sunnah to instruct and reassure the child after his burial?

The signs of a child reaching puberty are mentioned in Fatwa no. 34850. The funeral prayer for a child and supplicating for him is the same as the prayer and supplication for an adult, even though the child is not accountable, and the Sunnah has confirmed this. In Al-Muwatta’, it is narrated that the Prophet (peace and blessings be be upon him) prayed the funeral prayer over a young boy and supplicated for him, saying: "O Allah, protect him from the punishment of the grave." As for instructing the deceased after burial, it was not a well-known practice among Muslims during the time of the Prophet (peace and blessings be upon him) and his successors. The madhahib differed concerning it, ranging from being recommended, to disliked, to permissible. Its details are in Fatwa no. 31031.