Ikdry, I don't think you are responsible to drive him, make a list, or find alternatives, etc. But we all know that sometimes especially with people that we will continue to have a relationship with, we do things to make the relationship if not get better, at least not get worse.
Because of this I think you might want to consider making a list of phone numbers of taxi's, find a local college/high school student that might be willing to drive him (with him paying of course). You could even talk to people who are babysitters for children, maybe they would be willing to "babysit" your dad.
Print the list, hand it to him, wash your hands of the situation and then make plans with your kids for the day of the babtism. Do this so you can clearly say you are not available and cant be bamboozled at the last minute to drive.
If he is not able to go maybe the family members will learn that if they want him there they will need to consider his special transportation needs. Sometimes people need see the consequences so the can make better plans in the future.