How can one respond to a nine-year-old child who asks about the reason for prayer and doing good, given that God has already decreed everything and humanity's destiny in either Paradise or Hell?

It is true that Allah Almighty wrote everything before He created us, including the affairs of our lives. However, Allah created causes and linked them to their effects. Just as we do not accept injustice or deprivation by relying on destiny, so too we should not abandon good deeds and commit evil deeds by relying on pre-ordained destiny, because Allah made good deeds a cause for entering Paradise.