If your child gets ill on long car rides, for example, you might consider splurging for an airline ticket rather than making that cross-country trip. As much as possible, keep your child on deck where he can see the horizon. Don't let your child read in a moving vehicle. Open windows whenever possible. Avoid bringing along items with strong smells, such as greasy food, which can contribute to nausea.
Several drugs can prevent motion sickness, but none works every time.
Unfortunately, there's no way to know in advance which one will help your child, so if one fails, you may want to try another. All of those drugs are antihistamines, a class of medicines sometimes used to staunch the flow of runny noses. They all come with some side effects as well, which can include dry mouth and blurred vision. The most common complaint is drowsiness, so don't combine them with any other sedative. They work best when taken at least a half-hour before the motion begins, and some are available in special preparations, such as syrups.