Pack lunches and snacks, as everyone else has said. And don't buy bottled water, there or anywhere. Way too expensive plus an environmental nightmare. Stainless steel bottles with a few ice cubes in it to keep it cold and also serve as an ice pack for sandwiches. Put an insulated lunch box in everyone's backpack - everyone is old enough to carry their own stuff.
Make sure the snacks contain something with protein in it - e.g. almonds or peanut butter. Not just sugary snacks to fire up the blood sugar levels (and then crash, making people hungry) or salty stuff making everyone thirsty.
Kids will want souvenirs - another budget buster. Give the kids a set amount of money and suggest that they look around all day, making a decision at the end of the day when you're on your way home. That way they won't have to carry their purchase(s) around all day and will have a chance to look as you go through the park. There are many stores and stands, so it's not like they have to buy the first thing they see.
You're a Floridian so you know what to expect for weather. It's hot and humid. You'll see a lot of those personal fans or spritz bottles for sale so if you think it's going to be an issue, bring your own. One thing that's a big seller at WDW is the ponchos for those surprise afternoon showers. You might stop at the dollar store or even the camping section of a major retailer and pick up a few of the $1 emergency ponchos. If it pours, you can keep going without everyone complaining about the rain. You can also sit on an unfolded poncho if the rain stops but the benches are still wet. Even if the poncho can't be re-used, it was only $1 somewhere so it's no big deal.
And have everyone wear really comfortable sneakers with good support. No flip flops. Feet get sore, everyone wants to sit down, and that leads to whining about a sit-down meal and overall fewer rides - given the cost of admission, you want to see as many things as you can.
Map out your proposed rides ahead of time, allowing for preferences and height restrictions. One of the great tips we read about years ago in the "unofficial guide" was to go to the back of the park first, rather than jump on line for the first attractions near the gate. Walk past all the tempting rides and stores, and go to the farthest ride you are interested in. You'll have fewer crowds. Then slowly work your way back toward the gate. When everyone is exhausted at the end of the day, you'll be near the exit and won't be facing a huge trek across the park. Also read up ahead of time on anything that may be undergoing renovation so you don't waste time going there only to be disappointed and having wasted time.