I am sensitive to medications and didn't do well on Dramamine. I haven't tried the medication you are asking about, but thought I might make a suggestion that worked for me. I get VERY ill on the plane if there's turbulence, and can sometimes get sick on rides too. I bought Sea Bands for our October trip, and they worked wonders for me. They are little cloth bands you strategically place on your wrists that have a little button on them that is flat on the outside and rounded on the inside. Once in place, the little button pushes on a specific spot on your wrist to help prevent nausea. Pressure points and all that... If the nausea starts, you can press the button for a couple seconds and it calms down the nausea almost instantly. They worked so well for me that I bought some for my son who was refusing to go on Expedition to Everest b/c he gets sick sometimes, and my daughter who gets ill A LOT. I found them at Target in the section where Dramamine is for $6.00. They are washable, reusable, and completely safe. Plus they are guaranteed to not make anyone sleepy as they aren't a medication! You can always bring a medication as backup, but I would try the most natural method first for a child if you're not completely sure about a medicine.
P.S. I saw the Sea bands used on the TV show 19 kids and counting b/c their family gets motion sickness really badly, so if that helps....