On a recent trip home to see my parents, we got off the highway, where the kids were pretty much OK. But then, they started to turn green and my husband turned around in his seat to hold a bag in front of the kid behind the passenger seat. A few minutes later, both kids began an extravagant dueling barf session that covered them, their car seats, and a good portion of the car.
Since that dark day, I've become an expert in speedy car-sick clean-ups, and I can tell you that a few tricks and tools make the kids less likely to throw up in the first place, and the clean-up easier when they do.
Time Your Trip and Curate the View
Travel during nap time or after bedtime - if your kid isn't looking out the window, the motion sickness might not happen at all. We keep the windows open and the car cool - something about the wind rushing into their faces helps.
If you have a shade on your kid's window, pull it down to force her to look out the windshield instead of the side window. If it's safe to do so in your car, put the car seat of the likeliest-to-be-sick kid in the middle to encourage them to look out the front instead of the sides.
For us, Dramamine, which has a sedative effect, works if we administer it exactly an hour before our trips, which is usually also an hour before the nap. This means we give them a dose at 11am, feed them a light lunch, and then leave at noon, so they conk out for at least an hour (and the first hour of our trips is usually stop-and-go city traffic, which rivals mountain roads for inducing nausea).
Benadryl is also an option, depending on how your child responds to it, as "excitability" is a side effect. (Irritability is a side effect of Dramamine for my kids, for that matter, which means we save it for long trips.)
Some parents swear by ginger lollies, if the drugs aren't working. If your kid is over 10, you can ask your doctor about Scopolamine or Hyoscine, which is administered as a patch and doesn't have a sedative effect. As you should when considering any drug for your child, talk to your paediatrician.
Bags and Buckets
I have found these barf bags to be a lifesaver in that my almost-four-year-old can now hit them by himself (and they come lined with an absorbent pad for no leakage). Another option are these kind of bags, into which I've personally vomited after various surgeries, and are easier to hit than those face-scratchy rectangular bags you find on aeroplanes. Other parents swear by a bucket system, like a large yogurt container lined with a bag, or even a gallon bucket for a kid who's still working on her aim.
Before my son could reliably hold his own bag, I had a car seat cover, like this one, which made clean-up a lot easier. I bought it after a particularly gruesome hummus incident in which my husband suggested that we just throw away the whole car. These disposable ones look useful, too.
That doesn't take care of the straps, which are difficult to clean, especially when you are standing on the side of the New Jersey Turnpike swearing and wiping up vomit with old Hardee's napkins.
I myself am a big fan of the car smock - this one is $11, large, and has a pouch that theoretically will stop the river of puke from heading down to the car floor. I am super cheap, so I use a homemade smock: Basically, cut out the back of an adult t-shirt, but leave the collar and sleeves intact. Once kids are in the seat and strapped in, put the collar over their heads and their arms through the sleeves. The rest of the fabric tends to bunch in the middle between their legs, so it's not a perfect solution, but it does have the advantage of being free (and you can keep a few in the car, in the event of multiple incidents in one trip). This summer, I'm trying binder clips on the sides of the car seat to keep the fabric spread out, which likely won't work, and I'll break down and buy the $11 smock.
For protecting your car, you might want to put a seat protector under the car seat, or even place an absorbent doggie wee-wee pad underneath. (This is also helpful for those extravagant newborn/infant diaper blowouts that somehow drip through the entire car seat.)
Want to know more about what causes motion sickness and find advice for how to properly clean a car seat? Check out advice from mums who are also doctors and the Car Seat Lady. Happy (non-sick) travels!