I got the wrist bands and gave my 4 year old some chewable dramamine (after checking with her ped that it was okay). Both seemed to help. Also, make sure you keep something in their stomach but NOT a lot of liquid. Crackers, pretzels, toast, things like that - liquids will tend to slosh around like the seas!
Depends on the cruise and seas too - you may not have a problem. We left out of Baltimore and the only time she (and I!) didn't feel well was the day or so after leaving the Chesapeake Bay until we got around Hatteras and it smoothed out a LOT.