Is burying a child with the deceased considered a good deed or a good omen for him, given that the act was unintentional and the questioner was ignorant of the ruling on the reprehensibility of burying two in one grave?

The burial of a child with an adult deceased has no relation to a good ending (husn al-khatimah) or an increase in good deeds. It is not permissible to bury two people in one grave except out of necessity, according to the consensus of jurists, based on the practice of the Prophet, peace and blessings be upon him, and his Companions.