There are some good packing tips under the Royal Caribbean section of this chat room - click on the lefthand column first, then look down the list for the topic "tips on packing for a 7-day cruise).
The kids will probably enjoy Camp Carnival, the kids' program - check it out when you board, if you haven't already.
With kids along, I would recommend booking shore excursions in Belize through the ship - make sure to check the list of offerings and sign up early for what you want, as the popular ones often sell out.
Have a GREAT time!!! [img]/infopop/emoticons/icon_razz.gif[/img]