You're going to have such a great time taking them -- it's so special seeing DW through the eyes of someone who truly "believes." We took DD for her first trip when she was 3.5 and had a wonderful time . . .and she remembers as much of it as I do!
The thing to keep in mind, IMO, is that you have to tour on the kiddos' pace. Our daughter was an early riser, so we were at the parks at opening, stayed till around lunch-time. then back to the hotel for a nap, and back to the parks in the evening until a reasonable time. It worked like a charm for us. The only time we didn't leave to take a nap was the day we wanted to stay for the afternoon parade: instead, we left after the parade for dinner with Cinderella at 1900 Park Fare at 5:00, and were home and in bed by 7:30.
There's nothing sadder than a sobbing toddler who's parents are forcing her to have more "fun."
As for photo ops, you'll have a million of them! I'm going to assume that there's a good chance your DDs are princess fans, so be sure to plan a princess character meal -- pick from Cinderella's Royal Table, Princess Storybook at Epcot, Cinderella again at 1900 Park Fare, Alice in Wonderland tea party at the Grand Floridian, or if you want to go all out, the My Disney Girl tea party with Sleeping Beauty at the Grand Floridian. And if you do go to a character meal, be sure to let your little one dress up in their princess dresses, too. Also, Downtown Disney has Bibbety Bobbety Boutique, where the girls can get their hair and/or makeup done like a princess.
In the parks, you'll have more princess meeting opportunities in Mickey's Toontown, and of course, Ariel in her grotto. There's also Storytime with Belle in the MK, where they pick kids to go onstage and help Belle reenact the story of Beauty and the Beast. And at AK, if you can get your girls onto a first-row bench, they'll probably get to participate in the Festival of the Lion King. Our DD loved the rides, but the meeting characters was just as much of a thrill, so we made sure to check our Times Guide and hit as many of them as we could. Have fun!!