Being involved with Girl Scouts myself, I agree that the "cheapest" way to stay would be to rent a cabin. You could do tent camping, but with the extra amout of work that it takes probably isn't worth the savings since you would cut into FUN time.
As far as cooking goes, make sure that you rotate who is doing what so they feel they have some free time. Also, let them plan the meals before you go. This way they get the kind of food they want and if they are aware of the food budget it will make them realize that you won't be able to eat out all the time. Have them pick a few places that they really want to eat at or do a character breakfast at and schedule it into the meal plans. It costs a little more, but it is fun and just means that you cook an extra meal at the cabin. I had girls plan meals for a big camping trip and they did great with a little help understanding the cost of food.
You can buy the book Birnbaum's Walt Disney World and let the girls look through it. It has a description of all the different attractions, eateries, etc. There are also maps that will give them a better idea of where things are located so you can plan your days. I would suggest that they come up with a couple of "must do's". If you can, do for each girl so she doesn't feel left out. Then make plans to definitely do those things. After that, anything else that you do will be a bonus.
MGM will probably be of great interest for girls this age. Mine enjoyed the Tower of Terror, Star Wars, and the Back Lot Tours. If you get a park hopper plus pass then you can go to one of the water parks or Disney Quest. The number of times depends on the number of days your pass is for.
The Magic Kingdom is fun for all ages and the girls would probably love to shop on Main Street. Make sure you stay for the light parade at night. It is really beautiful and was enjoyed by my DD's
A great souvenir for all is a Disney autograph book (about $10). Even pre-teens will enjoy getting autographs from the characters and having their pictures taken with them.
At Epcot, they will probably enjoy going to the World Showcase. There are so many different little shops in each "Country" that are filled with items from around the world. It is really neat just to get a feel for the countries and see the differences. A great souvenir here is the Disney passport (about $10). You can take it to the different countries and they will stamp it for you. Sometimes they even sign little messages in the native language of that country.
We didn't make it to Animal Kingdom, but plan to this year.
My family went to the parks in the morning and returned to our room to swim in the afternoon. It also gave us time to regroup so that we could go back in the evening. This way you can enjoy the spectacular fireworks displays at the different parks.
Also, the girls might enjoy getting the same Disney shirt which you can buy before you get there.
And of course, never forget the number one thing on a girls mind. SHOPPING! They will love to shop or browse the many stores at Downtown Disney.
Hope this helps and hope you have a great time!
