I'd certainly advise not having dogs loose in the car, apart from it being a requirement that dogs are secured properly so that in event of an accident they don't become missiles. A previous colleague had a Lab loose in the back of a Discovery, with dog guard. One day after their caravan wagged the Disco and overturned the lot the Lab escaped out the smashed back window and ran frightened along the central reservation. Let alone my neighbour who let her King Charles be loose in the car, because it sat on the seat good as gold, until the time it got under the pedals.
One of our dogs gets travel sick so we give him a Stugeron tablet. Before, we used amazingly expensive tabs from the vet (25quid for 3 or 4 tabs plus consultation fee making about 50quid the first time, and found later you can't give them consecutive days) before we found info online about human anti-sickness pills. Ours are Collie types (~20Kg) so a whole tab (15mg is kids or adult human dose) is appropriate, for a small dog I'd imagine half or even quarter would be more appropriate. We were concerned about toxicity/poisoning/etc our dog from well meaning but possibly misguided info. However Hubby found an academic paper where they gave Beagles massive doses of it to see what happened, all was well. Tried it one day when we weren't going out and he was fine. So now we give him a tab with a smear of cheese on it an hour before we set off, he doesn't throw up: he's happier, we're happy... Not that this is veterinary advice, it's worth as much as you paid for it!/etc.