I donated to my Dad when he was 75, and nearly six years later his kidney function is terrific. I understand the 'fear of guilt' you are experiencing first hand. I chose to recognize it was present and move on without obsessing over it.
There is a small but real risk associated with every major surgery. There are risks associated with decisions we make every day, including driving the kids to school, going to the dentist, or eating in a restaurant. But I don't think you are facing a choice between your children and your dad -- might that thinking exaggerate the risk and attach a lot of emotional weight?
Perhaps if you research both the benefits and the risks you will be able to think about donation more clearly?
BTW, have you been testing for PKD genes? You do want to know if you have autosomal dominant PKD genes. I think that's the true risk you need to consider. Fortunately, it is one that can be assessed with a test. If you haven't had genetic testing, I think that is where you need to start. Actually, I'd want to do that whether I considered donation or not.