This is a very common problem—and one that can be very messy and unpleasant. It's also a problem that doesn't have a straightforward solution—except for avoiding car rides, which isn't always possible.
There are medications such as diphenhydramine or dimenhydrinate that can sometimes help with carsickness, but they can make children very groggy or sleepy. In some children they can have the opposite effect, which isn't helpful in a car either. Before you try any medication, you should talk to your pediatrician.
Simple Things You Can Do to Help Prevent Carsickness:
Have your child sit as close to the front as is safe (not in the back of a minivan), preferably near a window so that they can see outside and get a breeze
Have your child eat frequent light snacks, such as crackers or pretzels
Make sure your child stays hydrated, and avoid sodas and caffeine
Keep your child's attention outside the car; avoid tablets, video players, books, or other activities that require close attention
Use distraction, like songs or "I Spy" games
Keep the car cool and your child lightly dressed; being hot makes carsickness worse
Try to schedule long car rides for a time when your child would be asleep
It's also a good idea to dress your child in clothes that are easy to remove, and to bring along spare clothing. And…keep plastic bags, wipes and towels in the car for cleanup and to bag vomit-covered clothing—before everyone else in the car starts feeling sick!
Additional Information from HealthyChildren.org: