Some of our best Disney vacation moments were absolutely unplanned and happened by mistake. We ended up in Fantasyland for Wishes. And while we did swivel a few times in our seats, getting out of the Magic Kingdom was much less stressful than if we were in the thick of it. Also, instead of sitting for an hour and half on Main Street we let the kids play in 100 Acre Wood and rode Cinderella's Carousel and Dumbo a few times. Great night.
We also were just walking around MK one day before we left (mainly because DH didn't want to stand in anymore lines) and decided to sit down and get some ice cream. The Dapper Dans came out and started a set right then. For about 10-15 minutes we sat in the shade with great entertainment with our kids and some ice cream. Perfect moment, and now when I look back on our vacation that memory comes back to me immediately.
My point, while planning is great, and I have the spreadsheets to prove it. Sometimes the best moments in our vacations happen because things don't go according to plan. Relax, take it all in, and have fun. If you don't get there at rope drop, or hit Dumbo at the perfect time, or miss your spot for Illuminations, you will still have a great vacation. Go with the flow. Because the harder you fight the worse off you will be.
The perfect family vacation at Disney World, is less perfect and more family. The more time you spend focused on them and the less time on making it all go off without a hitch the more fun you will have.
I also second Cheshire Figment. Don't try to see it all in one week. Make a list of Need to See, Want to See, Can Live Without. Prioritize. There are certain favorites that we have to go on each time. Then there are others that can wait until my kids are older, etc. Even if your kids play in the water jets for an hour they are still having a great time in Disney World. True, you aren't seeing all that you probably would like to. But, you are having a fun, and doing more here than you would at home.