What is the expiation due upon me for running over my six-and-a-half-year-old son, who died instantly?

There is no difference in the ruling between the deceased being old or young. The death of a child in a car accident is considered accidental killing, and diyah (blood money) is due from the ‘āqilah (male relatives on the father’s side) to his heirs. The killer does not inherit from it, and expiation is due on the killer. There is no sin on the killer in this case, due to Allah's saying: {There is no blame upon you for that in which you have erred but [only for] what your hearts intended. And ever is Allah Forgiving and Merciful.}