I read somewhere that something in gingerale, has a calming affect on your stomach when traveling. I was looking up travel sickness tips too and this came up.
We had a 21 hour trip and brought cans of gingerale and snacks like crackers with us. Never opened the cans because we actually forgot we had them and my kids never felt sick. We stopped frequently, stretched, bathroom breaks and water was a must.
I think if they don't keep looking down for an extended period of time, that will definitely help.