Great advice given above. If kids get car sick they May get motion sick on a cruise.
One of my grandchildren seemed to get motion sick starting at about 2. She wears her "majic bands" (children sized Sea Bands) and whether physiacl or psycological, they work for her. Sea Bands are available in all the discount drug store chains in the same area that sells Dramamine and Bonine.
Personally I would try this as a remedy along with natural remedies such as ginger (pure ginger , cookies , ginger ale) before medicating a child.