Get one of those backpack purses, or even one of the small backpacks (at a discount store before you leave). Bags aren't a problem on rides in WDW (as long as they aren't HUGE). Be sure to take some Ziploc bags (we had the Disney princess ones from the $ store here at home) and you can put some snacks like pretzels, gold fish, your granola bars etc in them and even some hard candy to wet your whistle or keep the sweet tooth happy.
Some days we ate more than others but the portions there are huge....
They have these potato skins (chips) with all the fixins on them and they were so big 2 could easily share for around 4.50-5.00 and are something most teens like. The water is a good idea, but also I took bottled water in one of those bottles that has a strap to carry and put my favorite powdered drink in it. Sodas aren't terribly expensive ($2+) but are hard to finish when you are jumping off and on rides, so if you get them share with friends or get the ice water drink it, then split up the soda.
Club Cool in Epcot? You can drink enough free soda of various flavors to float the fleet.
The shrimp platter (about $10) at Columbia House in MK is HUGE lots of shrimp, plus coleslaw AND french fries. We both left at least 4 shrimp, were stuffed and wished we had shared. We also drank just half of the soda we bought and dumped the rest before going on Splash Mountain.
You can eat on the cheap. My suggestion is to eat what fills you up and is budget friendly then if you want to treat yourself with something special, like a cupcake, premium ice cream bar , a treat from the caramel shop in Epcot or something like that you can. One day I had a Mickey Ice Cream bar for Breakfast and another day, Chili cheese fries (well it was breakfast in my time zone and my first food of the day)! So anything goes.
Also be on the lookout for coupons. We had one to Earl of Sand. for a buy one get one free Holiday Sandwich.
If you're like a lot of people (and you sound pretty perky

) you'll be so excited to be there and do all the fun stuff that meals will slip your mind!
Just remember to keep yourself well hydrated, THAT is the most important thing. THAT and having the time of your LIFE!!!!
Oh, and this is just MY opinion, the food at Disney isn't to die for and worth spending tons of money on. It's mostly a survival thing and the occasional very good item (like something from the bakery in France at Epcot).