Second bringing individually wrapped sealed packaged snacks (or drinks) they like. For example, juice boxes or capri sun. When we went with my cousin's family many years back, we had some bartering on the high seas - trading juice boxes for diapers. Ha! My other tip would be to bring whatever medicines they could possibly need (children's tylenol, benadryl, etc) as they don't really sell children's medicine on the ship. Hopefully won't be needed, but a small quantity brought onboard is good peace of mind.
Something fun - the walls are magnetic so if you bring the kids something magnetic like alphabet letters or animals, they can stick them to the wall. Also download some of their favorite shows to your device as they may not have TV they like on the ship. A tablet or device with some of their faves can really help get through some of the longer dinners.