You are going to have an absolute blast. First do not worry about entertaining them. They will take care of that! We went a couple years ago with our youngest, then 13 years old. (Our oldest at the time was in Ireland with his college band, so no guilt about not taking him.)
Have your basic plan set, which days, which parks, and reservations. If they are adventurous eaters plan a few lunches at the Epcot restaurants. They will enjoy that. Plan at least one dinner where you stay at your resort and eat there, order out pizza...
Every morning, or before you leave your home, dial up the Steve Soar's website so you know about all the special stuff, and the times. Have them each pick an "extra" thing to do at the parks. For instance our son chose meeting "Push" at MK. That turned out to be the most adorable interaction. Keep in mind it took us awhile to find Push, and I actually ran up to him and almost hugged him gushing, "Push we looked everywhere for you!! I'm soooo glad we found you."
If Push could have blushed he would have, and DH and son were laughing so loud at me they were almost doubled over.
Explain to the girls to let you know when they are wiped out, or need a break. At around 2:00 each day DH and our son were beat. We stayed at a private home with a pool and we'd go back and hang out. Both DH and our son had no want to go out at night, which made it a very laid back trip.
Your girls might want to sleep in. Maybe you can agree to the first two mornings rope drop. After that let them sleep. You can hang out at the pool, take a walk... Be sure to bring a good book. Then head out to your park, grab a fast pass, and go for your reservation lunch. Grab another fast pass, and go to some rides with shorter lines. From what I've heard on this board the sleep in vacation plan can work. You can ask how others accomplish this since we are a rope drop family.
Give the girls time alone to hang at the pool, or drop them off at Downtown Disney to shop and eat lunch on their own. Girls will need time to be just the two of them. (You will also need a break.)
it's going to be a blast!!! There is such a difference bringing older children, versus the little ones, and you will truly enjoy the opportunity to slow things down, watch Off Kilter, or watch the performers at AK, or sit and have a beverage in a shady area.