First is finding out if you are even allowed to have them, so many places do not allow pets period!
We don't have the best car riders in our family. Crosley has a piercing whine, Camden gets car sick and Wrigley cries like a baby! So meds are a must if we travel any distance!
Do you crate them when you are gone? If you are concerned about accidents, I would contain them in someone's condo, otherwise you might be facing charges for clean-up, etc.. Not to mention if they have any kind of anxiety and chew on anything!
I would say every couple of hours you would need to stop, which you probably want to do for your child anyway to let them run around and get some fresh air.