Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you'd like to give your kitties something before their long car ride.
What to give will depend somewhat on whether they suffer from carsickness (nausea) or anxiety.
Either way I recommend all kitties be in separate carriers with absorbable pads so they are not upsetting one another, and should one kitty have an accident at least they all won't be sitting in it.
I recommend spraying the carriers and the padding inside with feline calming pheromones like Feliway a few hours before they are put in the carriers so it has time to dry.
I do recommend that they not eat for 8 hours before travel if possible.
Most cats tend to do better if they cannot see so I recommend a light cover over the carriers and putting the carriers on the floor of the car so they can't see out of the window and are jostled less.
If you believe any suffer from motion sickness you can give Gravol also known as Dramamine (dimenhydranate) for carsickness. The dose is 4mg to 8mg per kilogram (2.2 pounds) of body weight. So a 4 kilogram cat could take 16mg to 32mgs safely. Give it 2 hours before travel. It will last about 8 hours. Side effects are mild sleepiness and dry mouth.
Or if you believe that the primary problem is stress related you can try Benadryl (diphenhydramine) at 2mg to 4mg per kilogram of body weight orally. A 4 kilogram cat can take 1/2 of a 25mg tablet safely. Side effects are sedation and dry mouth as well. It should be given 2 hours before travel and should last 6 to 8 hours.
Neither of these medications are true sedatives, they simply have sedation as a side effect, so they may not be as potent as you would like, but they are the best available without a prescription.
Best of luck with the move, please let me know if you have any further questions.