What can I do about my dad hitting the dog?

I would tell them to stop immediately or you'll call the SPCA. I understand that it is a hard thing to do but your dogs deserve better than how they're being treated. What they're doing IS animal abuse and, once you move out on your own, you can get new dogs and treat them with the love they deserve. For now though it is best for the animals if you call the SPCA asap and get those poor animals out of there.