Well... are they just vomiting in the car or what exactly are they doing?
Meka's had issues in the car lately. In the past two months, we've gone for several short distance and two long distance (1 hr) trips. She always shakes from being nervous... but on the first hour long drive, she shook more than usual and vomited. So the next time, I waited to feed her until we arrived at our destination... and that prevented the vomiting. Then I usually held her in previous trips, so this time she had a carseat... though she still shook at first, but midway there she was a lot calmer and did much better than the last.... She's 8 mo's right now...
I'd say try not feeding them (until later) if they're vomiting... or the pet carseats
