Hi! I'm 13 and am about to embark on my 5th cruise over spring break. Personally, I have never had problems with sea sickness, and neither has my 12 year old brother. The only time we had an issue was when we were OFF the cruise ship to take a very small motorboat across a body of water in Cozumel. They gave us swabs of alchohol to sniff, but I of course didn't sniff it, because it kills brian cells. Instead, I rubbed it on my arms and was fine. I've heard of some people taking ginger pills a few weeks before the cruise. If your kids aren't big on taking pills, if they keep themselves occupied (which is never impossible to do on a cruise), they will most likely be fine. Also ginger ale and carbonated beverages always help me. I sometimes take dramamine too.
Good Luck and Happy Cruising!