If you do decide to take medicine with you, try it out first. Some kids may get really sleepy; others may get wired, which is even worse when you're stuck in the car. Benadryl will also work, since motion sickness meds are antihistimine, so keep that in mind.
Have your kids gotten motion sickness before? If they're usually okay, I wouldn't worry too much about it. Just take some common sense precautions, like not too much reading, eating light food, and taking frequent breaks. Sunglasses are great too if it's sunny when you're traveling.