You can buy phenergan for travel sickness and it works very well but does make them drowsy. Have you tried other travel sickness pills ? Phenergan is hard to buy in the states as it can be used to increase the hit from codeine. I think that we can get mostly the same antihistamines but they are just called different things.
I was prescribed phernegan for hyperemesis when pregnant so it is definitely used for antisickness, I found it very effective. However it did make me very drowsy though so couldn't take it very often. Saved it for DSs nursery days!! It is prescribed for children, I remember seeing it on the dosage guidelines which I read numerous times, a quarter or half tablet depending on age IIRC Fairly sure prescription only though HTH
My GP prescribed it for my DS1 for his insomnia. Be warned it had the opposite effect for the first hour and sent him wild and very energetic till he would finally yawn and drop down asleep. Plus it only lasted half the night so we still had the second half of the night with him awake!