What is the ruling on the prayer of one who prays behind an imam with a child, but they do not form a row, and is his prayer considered as if he prayed alone? And what is the appropriate age for a child to lead people in prayer as an imam?

If a boy stands in the row with a praying man, the man is not considered to be praying alone behind the row. As for a boy leading an adult in prayer, the majority of scholars believe it is impermissible, contrary to the Shafi'is who hold that the imamate of a discerning boy for adults is valid. They cite as evidence the imamate of Amr ibn Salamah when he was six or seven years old.