What should a Muslim do if while driving he accidentally hit a ten year old non-believer who eventually died?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If a Muslim kills a non-Muslim who is not in the state of war with Muslims by mistake, he has to pay the Diyyah (blood money). There is no disagreement over this ruling.

The blood money is given to the family of the deceased. If they remit it, then the killer is not obliged to pay any compensation. If some of the family members of the deceased remit their share of blood money, then he still has to pay the other members their rights.

The blood money for a non-Muslim is fifty camels or their value in cash. ‘Aaqilah (all the blood relatives from the father's side) of the killer are responsible to pay that blood money, and everyone from them has to share what he is capable of giving.
Is there any expiation for killing a non-Muslim by mistake or not? There is a disagreement among the Muslim scholars. The preponderant opinion is that only payment of blood money is mandatory, and no expiation is due for such an act.

For more benefit on Diyyah (blood money) in Islam, please refer to Fataawa 384850, 91306, and 101103.

Allah knows best.