My very first action would be to make sure that the children understand that they are not to climb on, over, or between any railings anywhere on the ship under any circumstances whatsoever. The risk of a child falling either overboard or onto a hard surface two or three decks down is one that I'm sure you don't want to take.
After that, a lot will depend upon the time when you go aboard. If it's time for lunch, by all means eat on board. If time allows, you can go for a swim or do some sightseeing in the port. I also like to find my way around the public spaces so that I have some idea where to go when the activities start. If you have some idea where you and your husband will hang out during the day, it's also a good idea to familiarize your children with those locations so they will know where to find you if there's a need. Additionally, you undoubtedly will want to unpack. By then, it probably will be time to get ready for dinner....
Anyway, do have a great cruise!