Wow, six is such a magical age. She's going to have such an incredible time, and so are you.
Sit down with your daughter with guidebooks prior to the trip. Help her understand the descriptions of the various attractions, and together you can compile a must-do list for each park. However, I would not waste time at the park criscrossing back and forth to get to the must-do's. I prefer to pick a starting point and direction of travel, and do attractions as we pass them. The only exception is for FastPass rides...but even then, I would pick up the FastPass and do the other things in the immediate area while waiting for your FastPass time. Your must-do list will help you decide whether any given attraction is worth waiting for.
Of course the MK is most kids' favorite, and many adults too. But there are plenty of attractions in the other parks to keep her busy and happy as well. Depending on your daughter's temperament, she may not want to spend a lot of time in the World Showcase at Epcot. But there are KidCot stations all over Epcot, with fun activities to keep the kids entertained. Seek those out from time and time, and that should help break up her monotony as you tour the shops and grown-up attractions.
Depending on her interests and level of thrill-seeking, Future World should be fabulous for her, with Mission:Space, Soarin', Test Track...lots of fun.
If your daughter is a movie watcher at all, she will love Disney-MGM Studios. Voyage of the Little Mermaid, Beauty and the Beast, the Great Movie Ride, not to mention Tower of Terror and Rockin Roller Coaster if she likes thrills.
Lots of people enjoy hitting the parks early in the morning then heading back to the hotel for a swim and a nap before going back in the evening. If that works for you, go for it. We always found it to be a waste of time though. We much prefer to go hard from open to close on the "park days," and sprinkle in some scheduled "break days" where you don't go into the parks at all. That way you don't waste park time driving around or waiting for the bus.
You didn't mention how many days you are staying. If at all possible, I would try to plan for MK as the first park you see, and also the last. That way, if your daughter does grow bored with the other parks, you can always remind her that MK will be coming up again on your last day. If you have the park hopping option on your tickets, you can also do partial days at each of the other parks, and finish up your days at the MK.
All of the parks are just wonderful for kids. Your daughter will have an absolutely fabulous time. Make sure you get lots of pics!!