Wow, you have gotten some wonderful tips already!! My boys are 7 and 5. Here are some tips that have really helped us!
Take breaks! Talk to your girls and let them know to speak up and let you know if they are tired or need a rest. We usually take a break every 2-3 hours and have the boys eat a snack, and have them hydrate and use the restroom. We only drink water at the parks, with the exception of milk for meals. With both boys no longer using strollers, we will also take quick breaks if they get tired from walking. We average 7-10 miles a day at the parks. That's a lot for little legs. We also take long walks at home to get them more used to standing and walking a lot. It helps! Breaks also help how overwhelming the parks can be. It is loud, crowded, and there is SO much to take in. Having a break helps my kids from getting overwhelmed.
I will repeat what a PP said about waiting until the last day for a souvenir. A lot of the attractions will exit you out into a store (looking at you Buzz, Star Tours, and Toy Story!). The boys would ask for everything! We made a rule that they could look, but could not buy until the last day. That way they get what they really want. It totally helps! (we also limit one "treat" a day of their choosing)
You can get FREE water at the counter service restaurants. Free! And you don't have to carry around water bottles or pay $3 for bottled water =)
Don't skip Tom Sawyer Island! It's a great place for the kids to run around and get out some energy =)
Take lots of photos! They are only little once =)
I would try to get an idea of the places your family would like to eat in each park, that way when you all get hungry, you will know what is near and already know if your kids will eat the food! Disneyland.com has the menus up for most places now. So helpful!
I think Gadget go coaster is a great starter ride to kind of feel out how they will react to other attractions (like roller coasters). I keep things really positive in line. Telling them how big they are to ride, saying how much fun it will be, etc. I never ever say "scary." We have used bribes before to get my youngest to try an attraction (like he will get a sucker for trying). We tell him he only has to try it. For Big Thunder, he loved and went back multiple times. For Space mountain, not so much! But, he tried and was proud. Frank, the big combine tractor in Radiator Springs scares my youngest. He has been on RSR lots of times, but always asks me to cover his eyes on that part and he's fine. Also, even though he has been on Star Tours more times than I can count, he won't go on it anymore. He truly, truly believes we go into space. He will say, "I don't feel like going into outer space today."

So we don't push. My oldest is the opposite and has been on every ride in the park, including TOT and Screamin. He loves everything. I think you will be able to tell and will know your girls best when the time comes. =)
I also bring a change of clothes and socks for the boys just in case. Put them in a locker. We also bring in a ton of snacks from home. I try to get new items or items they haven't had in awhile. Even though we have them eat on a regular basis, it is hard because they just want to go, go, go. So having a new snack or something they don't get very often, really helps!
Lastly, enjoy! Take in the moments! Laugh, have fun and play with your kids! Don't sweat the small stuff. You will have a wonderful time!!!!