There is a drink station on deck 9 where you can get milk, fruit juice, punch, soda, tea, coffee, etc. all free, all available all day. Your steward should put ice in your room. The tap water is safe and tastes good. Bottled water is available, but is expensive. Pop, iced tea, fruit juices, etc. are all available in the dining rooms at meals too.
What to pack in your carry on--any medications, sunscreen, swimsuits (the pool is empty on the first day, so after check in and making reservations, it's a great time for a swim...you may not have your luggage yet.) Pack anything that you need before 5 pm on the first day and anything that you literally or figuartively can't live without. Camera too. Try to keep the carry ons light. You can't get to your stateroom till about 1:30, so you may be carrying them for awhile after you board.
Register the kids in the programming, and insist that they do at least one activity per day in the club/lab. That will give YOU a break. Hopefully, they will make friends and want to be in the programming, but if not, you still need some adult time. Pack light, remember that it is OK to say NO, and relax and enjoy! I think you should get the grandparent of the year award!