At any given moment, even at the magical WDW, you can look around a see a child in tears, or dragging their little feet and whining about something...
It is a wonderful, magical place, but it can also be a huge trap for the "gimmies," the "carry-me's," the "I'm hots," and the "I don't wannas."
Helping your kids know what their boundaries are as far as souvenirs (we'll buy them on the last day, or before we leave the park or whatever... so you don't end up lugging them around and trying not to lose them on rides) will help.
Flexibility will be your other key. Our first trip with kids aged 14 and 4 was almost a disaster because I was determined to follow the Unofficial Guides' 3 day touring plan if it killed us, and everyone was going to enjoy it or else!!! Realize up front that unless you are staying for a couple of months, you can't see and do even half of what there is at WDW, and try to choose before you get there what things are really important to your family. Then, on your way home, start planning what you'll see that you missed this time on your next trip, while it's all fresh in your minds.
Last, sometimes, stopping and eating a Mickey bar instead of seeing that show in 5 minutes is worth so much more to your sanity. (For sanity's sake, our kids ate lots more ice cream than they are allowed at home!) The show will still be there another time, but if you stop for a Mickey bar, the whining should stop (temporarily!)
You'll have a great time, and the good memories will erase the few bad ones!