When my children were younger travel sickness was something I tried hard to guard against, because vomit is one aroma which I am not partial to! My eldest daughter would often complain of feeling sick while travelling on motorways.
If she had been eating I would give her peppermint oil to smell, as peppermint is renowned for its stomachic properties (calming the stomach). Nervous tension is also a cause of travel sickness, and in this case the aroma of lavender would be more beneficial, as it is soothing and calming.
The easiest way to use these essences without fear of spillage is to put one drop of either peppermint or lavender onto a tissue, and breathe in the vapours. Only on one occasion have I needed to clean up the back seat of my car, and then it was my fault for allowing my daughter to eat a huge cream-cake in a motorway service station.
If a child (or adult for that matter) is prone to feeling car sick, the choice of foods taken before and during travel should be given careful consideration.
Lavender and peppermint oil on a tissue is the easiest way of breathing in the soothing vapours. A tissue impregnated with these oils can be placed inside a small plastic bag (sandwich-bag-sized) and popped into the glove compartment. When needed, just tear open a small hole in the bag and inhale.