Hi Francesca,
Don't worry! I have a 7-year-old daughter myself, so let me give you some ideas on where I would start planning.
I like your idea of having theme park tickets for only certain days of your trip. It is nice to be able to take a couple of days to relax at your resort, or explore other parts of Walt Disney World. If you would like to visit all four theme parks, I would suggest that you start with one theme park per day for the first four days, and then decide which ones you would like to revisit on those last two days.
I suggest that you order the
free Disney World planning DVD. Watch this with your family so that you can get at idea of what to expect at Walt Disney World. Then, have your daughters make customized maps of the Disney theme parks at customizedmaps.disney.go.com. These are free and will be delivered to your house before you go on your trip. Through both the DVD and the customized maps, allow each daughter (any anyone else in your traveling party, if you like!) to choose three attractions at each park that they really want to do. Once you know what are everyone's "must-dos," compile these into a list and this will give you the attractions to experience at each theme park. It is simply impossible to do everything at each park each day, so by letting each child choose what they really want to do, you set up everyone to have a fun day because their must dos will be done!
I also suggest at this time that you start to consider which resort you would like to stay at, as well as where you might like to dine. I am happy to help you with those questions as well; however, I am limited in the space that I have to answer, so please do send more questions if you like. This really will be a magical vacation, and you are smart to plan ahead to ensure that all goes smoothly!