How can one atone for hitting one's father in childhood, and will God forgive this great sin after the father's death?

Your sin is great, but Allah's forgiveness is greater. So, be sincere in your repentance, increase your supplications for forgiveness (Istighfar), give charity on behalf of your father, and maintain ties with his kin. Do not neglect your mother's rights or treat her poorly. Seize the opportunity of her presence to show her kindness and dutifulness. If her situation does not change, then you fulfill your duties of dutifulness and kindness towards her and your grandmother.