How can one repent for stealing from the ATM, a practice the father has become accustomed to, while fearing his reaction should the money stop or the theft be discovered?

The first step towards repentance is to feel guilt and remorse for betraying the trust and unlawfully consuming people's wealth. So, hasten to sincere repentance for the money you took from the shop by returning it, regretting it, and resolving never to return to such an act. If you do that, you will be absolved of it.

As for your father, do not inform him of this, and do not uncover Allah's covering over you. Excuse yourself to your father using whatever allusions and indirect statements you can. Perhaps Allah will provide you with what will meet your needs and your father's needs due to your repentance and your dutifulness to your father.