We took our 2.5 yo dd, almost 5 yo ds, and almost 13 yo ds to WDW for 10 days at the end of May. My detailed plans were shot before the end of our first day; but that was ok since I kind of expected them to be. I knew that dd and younger ds woud prefer MK to the other parks, but I didn't realize how much. We spent the majority of our stay at MK!! By the end of our trip, dh was saying, "Please, can't we go to another park?!!" But, of course, the little ones kept requesting MK. Of the other 3 parks, DS/MGM had the least appeal to my little ones. It was just too hot at the outdoor shows, the 3-D shows were sometimes frightening to our dd, and the crowds made just looking around nearly impossible. We did see far more characters at DS/MGM, but the lines to get autographs/photographs were extremely long. Other than that, there wasn't much for the little ones there. At Epcot, the different peformers kept our children entertained. AK was a big hit with all 3 kids but also extremely hot. We found that we could only stay a couple of hours at a time before we found ourselves just wandering around with no direction. In general, just too hot!
We did find ourselves splitting up several times so that dh or I could ride something with our older ds. Fortunately, our older ds likes to do things on his own. He had no issues with riding by himself. (I was definitely NOT like this at his age. I can't remember a single trip to WDW growing up without a PACK of friends along.) Getting reunited was often problematic for us. We each took cell phones, but tossed them into the same backpack. Of course, a cellphone does you no good when the person you want to call has both yours and theirs! Also, parades and other obstacles would interfere. After a couple of days, we began sending our ds to DQ on his own with a phone. He had to check in with us every so often, but this was never a problem.