My family loves Disney and we have done it with kids of all ages.
Yes it's "easier" to buy drinks in the park, but you will be blown over by the prices and the lines. It's far more sensible to have water with you, when you can drink it while standing in lines for events. I would suggest you take a metal (stainless steel) bottle for each person - you can refill as you need to, and you can add ice without it sweating as much as plastic. It also won't crack in a crowded backpack. (I also find people throw plastic away a lot more easily than metal.) Although only your husband is flying, he can easily take an empty container through security, then fill it on the other side and have it on the plane.
Otherwise, definitely buy something outside of Disney, not in the parks. Your kids are old enough that they won't care about a Mickey Mouse water bottle for $9.
We always took a small soft-sided cooler with a shoulder strap and ice packs into the park. It's absolutely allowed. Disney actually WANTS you to have a good time and they don't want anyone with heatstroke! Be sure you have a freezer or at least a mini-fridge where you are staying so you can re-freeze. You can also put some water in the metal water bottles and freeze, then top them off with plain water, and use that as a way to keep things cool (not just the water, but the stuff you put next to it. You can't freeze plastic bottles - not safe. Ideally, you can keep your own foods in the fridge - apples, etc., and take those along for snacks.
You are probably taking food in the car, so be sure you have a bunch of small containers you can re-use for perishable or squishable food - berries, etc. You can stop just before you get to Disney and stock up, and also get a very small bottle of dish detergent for washing things out.
Everyone should have a backpack, or you can have an insulated backpack that rotates from person to person for carrying. That can include a drink, sunblock and snacks. Disney has many restaurants, snack bars and push carts, but the money really really adds up, and your kids will think you are the First National Bank of Orlando if you keep forking over the cash. You can also spend a lot of time in lines for drinks, and many of them are sodas or sugary lemonades, which send you to rest rooms to wait in other lines. Otherwise, as someone else suggested, give each kid a daily allowance (for snacks, drinks and souvenirs) and make them make choices.
I agree about the Unofficial Guide - it gives updated into on parks, lines, trends, etc. One of the things we learned for one of our first trips was to enter the park of choice and head to the REAR - fewer crowds because everyone goes to the first attractions they see. If you start at the rear and work your way back toward the entrance, you're close to the exit when you are hot and tired and loaded down with souvenirs. For snacks, take some high protein things like almonds or trail mix, not just chips and salty stuff which make you even more thirsty. However, replacing some salt lost through sweat is a great idea.
There also used to be a "science" of which line to get in - someone had studied the psychology of whether more people chose the left line or the right, and then told you which was shorter. Don't know if that's still operational. The guide books also give you some good age ranges for different rides - your kids are well beyond a lot of what the Magic Kingdom offers but there are still some awesome rides, not just for the thrill of it but also for the special effects and creativity.
Weather is hot and humid, no exceptions. Showers are frequent -a lot of people use (and Disney makes a killing by selling) plastic ponchos. You can probably find some really compact ones in the dollar store - they are incredibly handy. I like the ones with the hoods. If it rains, people just keep on going at Disney, FYI!
You'll also need a sun hat for everyone and sunglasses, tissues, maybe sweat bands, and maybe those personal fans/spray bottles. Don't overload your backpacks but definitely prepare. You might also want a container of deodorant for touch-ups, and some plain wipes for getting the sunscreen off your hands (especially if the kids have their iPods or phones with them - sunscreen is not good on a touch screen). It sounds silly, but remember to really goop up the back, the neck and the back of the ears with sunscreen - those take the bulk of the sun when you are standing in line and turning your face away from the sun. No matter how "grown-up" and tough your kids act, let them know you are not ruining this special trip with a raging sunburn!
Other things we found helpful were a travel clothesline for hanging up wet bathing suits or rinsed out sweaty shirts/tops, and a few packets of travel-size detergent for washing things out in the sink.
You'll want sandals for part of the time at the hotel, but you absolutely need fantastic walking shoes or good sneakers for all the hiking around you will do. I've seen too many people in flip flops and sandals really moaning about their feet.
Have a great time!