You do have a few options available. Depending on what ship you're going on, they may or may not have a self-serve laundry. If it does, that can be invaluable. If it doesn't, then many ports have self-serve laundry facilities. Otherwise, plan to pack a suitcase for each child and take along many changes of clothes. Or take along some laundry soap and plan to do some washing by hand in your sink. Another option would be just to pay the fees on the ship to let them do some laundry for you, but that can be quite expensive.