1. Always keep a large cup in the car. It's even better if it has a lid just in case you have no place to stop and rinse it out once it's been "used."
2. Cover the window adjacent to the child's seat with a thick blanket to block out any motion in their peripheral. We've found that by putting Jackson behind the driver's seat we can tuck the blanket in such a way that the driver can still see out of his blind spot.
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4. I always keep a pack of gum and a few extra munchies in my purse when we travel. I know a full tummy helps my motion sickness, so I figured it'd help him too.
5. Now that Jackson is old enough he can tell us when he is starting to feel motion sickness and we can tell him to close his eyes until he feels better. Colin will also crack the windows for a few minutes for fresh air and that seems to help too. We've avoided having to use "the cup" several times this way.
Those are the things that have worked for us. I'd love to hear solutions from others whose kids struggle with motion sickness.