Regarding the bottled water, 3 cases would be 72 bottles which, divided by 9 days and 5 people, would yield 1.6 bottles per person per day. Yes, you guessed it, I'm an accountant, and numbers are my game. Anyway, as you can see, that may or may not be enough for your family. It depends on how many bottles each of you drink in average. So... to make it easy... just figure out how much each you drink per day, multiply by 9 days, add it all up and... voila! you'll know how much water you'll need. Do the same for the juice.
I bring Poptarts, bread, cold cereal and milk for breakfast. I keep the milk, juice and water in a collapsible cooler and I fill large ziplock bags with water every morning to keep everything nice and cold. I don't know where you are staying, but Pop had toasters at the food court, so one day we had cereal, another day we had Poptarts, and yet another I toasted my bread and brought some butter/jam from the food court. I bought one refillable mug which I used for my morning coffee and then lemonade for my DD during the day.
I also brought some beer for my hubby and me. For snacks, I brought Hawaiian bread, chips, prepackaged fruit snacks (they're like gummi bears but supposedly made out of fruit), grapes and cherries. I kept the fruit in the cooler as well.
Since we were going to Blizzard Beach one day, I also brought different cold cuts and we made sandwiches for lunch, with chips, and snacked on fruit (which tastes great after you spend a lot of time in the water) and Hawaiian bread.
It sounds like a lot of stuff, and it was only for my DH, DD7 and me, but we came back home with very little left. We came to WDW with 2 full coolers and several grocery bags, and left with only half a cooler and 1 grocery bag (which only had leftover sandwich bread).
Hope this helps.