I have way too much experience with sea sickness both in myself and others, including children.It's a tough time with kids and sea sickness. Most "childrens" meds are less effective and I would be hesitant on giving adult meds to smaller kids. One of the biggest problems with SS is that once the symptoms appear it is almost always too late, meds are not going to help. My experience especially with kids is that once it starts don't fight it. Go ahead, be sick, get it all out (sorry for the pun). Then get horizontal and get to sleep. You can actually sleep the sickness off, especially if the motion continues while you sleep. You should awake refreshed and even ready to eat, hopefully.
Another thing is to keep the blood sugar up. Sure, they might be unbareably hyper but they will be less likely to get seasick. You might also try the wrist bands with the little accupressure points. You can use rubberbands or bread ties to keep them snug on little wrists.
Here's hoping you have a great cruise. Bon voyage et non al le mal de mer!