We are late to bed late to rise, but we go often and don't feel the need to do everything. As this is your first time I would suggest, as others have said, to get to the MK early a couple of days. You have picked a very busy time for your first time (we've been during Easter) and you will get a different experience that you would at another, less crowded, time of the year. That being said, if you can do Easter you can do anything, trust me! I would avoid early opening days, that is don't go MK on the day it's early opening unless you are getting there early. Sometimes the day after is a good bet. Also late at night is good, as is during normal people's meal times. We eat at very different meal times than others and this has always worked for us. The weather is usually perfect at that time of year. Sometimes I am just happy to walk around in the good weather (but then I come from NE!). Avoid MK on Easter and avoid the water parks on week-ends (tons of locals) also DD can get crowded on week-ends with Friday being "date night" with the local teens, etc. You will have a good time especially if you go with a good attitude. Remember you can always do what you missed "next time"! OH and another thing, don't rule out some of the "thrill rides". You will be surprised what your kids may want to do once they are there!!
Have a Wonderful Vacation!!!