I haven't taken my two little ones on a cruise yet but have been on one and I strongly suggest taking any medicine you think you may need. The medicine in the ship store is insanely expensive. My best friend paid something ridiculous like $18 for a small box of Immodium.
Stuff we ended up needing that week:
Hydrocortisone, Mosquito repellant, Benadryl, Dramamine, Gas aides, antacids, Immodium, Pain Killers, sunburn aloe vera gel. Depending on where you are going I might even call the Doctor and ask for a Tamiflu prescription to have handy. Lots of germs floating around on cruises. Check with your pediatrician, the kids may need shots to travel.
You will need lots of sunblock. We ran out and it was super expensive as well.
We didn't pack water shoes and paid $20 per pair in Jamaica and they fell apart.
Umbrellas or panchos!
The passengers with small children didn't really join us on the outings but if you plan to, I strongly suggest slings for the twins as opposed to strollers. It will make getting around much easier.