A simple way to treat nausea is to put a slice of raw ginger in the mouth and suck it untill you feel better. If your child feels motion sickness you can press on the point between his or her eyebrows. This is the point Yintang, a well known calming point in the Chinese medicine, which the yogis call the "third eye".
Press with your thumb on the middle of the inner side of the forarm, two thumb's width from the wrist crease. Hold a firm pressure between the big tendons for about two minutes as you take long, deep breaths with your eyes closed.
Usually, however, the ailment is treated by calming the stomach area. Using the thumbs or the palms of the hands, rub and stroke your stomach along your rib cage from up till down. And since motion and cold are likely to alleviate the condition, you should try to keep yourself as still and warm as possible.
A complete acupressure routine for nausea, motion sickness and morning sickness contains a lot more acupressure points. If you want to learn more, order my book Treat Yourself with Acupressure at Amazon.com