Is it permissible to perform Salat al-Gha'ib (funeral prayer in absentia) to compensate for a shortfall in the number of takbeers in a funeral prayer I previously performed for a child, where I said three takbeers instead of four?

The funeral prayer for a child is obligatory according to the consensus of the majority of scholars, based on the hadith: "Pray over your children, for they are your forerunners," and due to the general texts regarding praying over Muslims. If the prayer was not performed correctly, then it is obligatory to pray over his grave. If that is not possible, then the absentee funeral prayer should be performed for him.