I wouldn’t leave them in same milk for 8 days. Better option is to freeze them in a little milk. They should travel well by car. You put them in fresh cold milk right before you leave and if you have a cooler even better. Same way they sit out on the counter, they can be in the car for 7 hours. Temperature is higher but that will just make the kefir a bit sourer. You can either finish ripening it until full 24 hours when you arrive or toss this milk and make a fresh one on the right room temperature.
Hope this helps.