What is required of a driver who unintentionally caused the death of a child hit by his car, knowing that the child's family has forgiven him from paying the blood money (Diyya)?

Since the parents of the murdered child pardoned the blood money, the driver must pay expiation for accidental killing, which is: freeing a believing slave. If he does not find one, then he must fast for two consecutive months, as Allah Almighty says: "But whoever does not find [such a slave] - then [expiation is] fasting for two consecutive months as repentance to Allah. And Allah is Knowing and Wise."