Do not let them run lose in the vehicle! Cool water and proper ventilation are absolutely necessary.
With a towel or blanket they will be snug and comfy…no need to get out. Even if the doors of the vehicle are all closed to prevent escape from the car they could hide under a seat and not come out, then later crawl under the brake pedal or something. They must have some sort of ID. Purrhaps, take pictures of them.
First, they need a secure enclosure that has proper ventilation in the space. I know they’re small and nap a lot but they need a space to move around. This space should have a small litter box. The kits will (most likely) play and sleep in the litter. If you don’t want the litter box, I recommend pee pads in the enclosure.
It would be good to make pit stops to give them (and drivers) a break. Do not let them walk around by themselves. You don’t know how easily kits can run off.
The only way a cat can cool herself is by drinking cool water. Water is an absolute must. They could bring some Goats milk. It’s good for digestion and has lots of vitamins. It would help keep them hydrated and fill their tummies, too. Goats milk is a good food for kittens. It’s easier on the tummies so they don’t throw up. The problem with solid food is it’s too heavy and when they get an upset tummy…
I think that’s all. Happy travels and you sound like an awesome purr parent.
PG