I was always car sick as a child. Not many things helped. There are special chewing gums that you can buy that are supposed to help, but the smell of them made me nauseous . My nana swears by putting small, strategically placed bandaid squares behind the ears (it's supposed to help with the sense of balance - the cause of travel sickness). If I remember correctly, they go just behind the ear lobes on your head (not the ears) You know, the soft spot between your skull and your jaw (Acupressure maybe?). This worked for some kids, but not all. The one thing that really helps is NOT LOOKING DOWN (i.e. reading or watching movies, playing psp etc), but to look out the window in the far distance (again, to do with your sense of balance or something ). However that does become boring when you're a kid. Trust me. Been there, done that.

My last suggestion is not always practical, but has helped enormously on those looong car trips (more than 3 hours). Driving at night. I could never sleep during day time car trips (too sick ... closing my eyes made me nauseous, too ), however at night time I was always fast asleep and NEVER got car sick. Beats me why... But hey, it worked!

Sorry, that's all the advice I can offer. From the perspective of the "sufferer"