I would try to get to the terminal between 10:30 and 11. This will give you all a chance to look around the terminal a little. You can find Castaway Cay on the big map on the floor so you can see where you will be on one of your stops. Also, there is a model of the ship in the terminal. And the real ship is outside. Lots of kids like to look at the model, then run outside and look at the ship (repeat as needed). THere are comfy couches in the terminal and many TV's showing Mickey Cartoons. Then the characters come out for photo op's before boarding. The 1 to 1 1/2 hours waiting in the terminal go by very quickly.
After you go through the Mickey ears to board the ship, you will be offered the opportunity to have an embarkation photo taken, so you might want to coordinate your clothes somewhat when you get dressed in the am.
Once on board, you might consider getting the kids signed up for the clubs as soon as possible. It doesn't take long, but there are a lot of families that need to get signed up so the line can get long.
If the kids like being in the pool, bring their swimsuits in your carryon luggage so that they can change into them and hit the pools before they get crowded (or the kids can wear their swimsuits under their clothes).
Keep reading these boards for lots of good info that will help make your cruise a family trip to remember!