My top two picks would be the Grand Class Ships of Princess and the Voyager Class ships of RCCL - these are the really big ships with lots of stuff and things for all ages. Both have very good kids programs and cater to families. Now for the hard part - where to go. Make a list of what you and your kids like to do on vacation. Is it just hit the beach and hang-out, or do you want to try something more adventurous like swimming with the Sting Rays, climbing water falls,snorkeling, visiting ruins and of course shopping. I would get some brochures for everyone to look at. It helps to have something in hand.
Accomodations - for familes we found that it was cheaper to book a mini-suite than it was to book 2 cabins. 4 people in a standard cabin is tight.- especially when it comes to the bathroom. 4 or 5 people in a mini-suite is comfy.
Soda cards/stickers worked great for our kids. 7 day cruise on Princess $22.50 for unlimited fountain sodas.
Coffee, Tea, Iced Tea, tap water and fruit juice is included in the cost.
The average price for alcoholic drinks is about the same as what you would pay for a drink in a nice restaurant back home. The fancy frozen drinks are more expensive, look for drink of the day specials.