We just got back from a week there, and I think it all depends on your kids. We stayed on property, and just spent the first day exploring the hotel and pools. The second day we did Magic Kindgdom - my kids are 5 and 7, and we went at their pace. The lines weren't actually bad at all for the kiddie rides, and my kids are too small to go on the larger ones, so that wasn't an issue. Take advantage of the fast passes if you can, they keep you from having to stand in line for an hour - you just use your park tickets to get them, then come back in the assigned time frame. The only ride we had to do it for at MK was the Toy Story ride, but it would work well for the bigger rides as well. Plan for $10/person/meal if you eat in the park. The kids get two sides with their meals, so they will fill up (and it's stuff like apple slices, grapes, carrots). It was much more convient to eat in the park than to bring a cooler, but got expensive quickly. I kinda had a heart attack everytime we ate :p You just need to figure out which works best for you. Wednesday we went to downtown Disney - there's an arcade there that has 3-d adventure rides which are a lot of fun, and it's a cool place to just hang out. Some great restaurants too. We spent Thursday at Animal Kingdom - if your kids like animals, they will love it. There are also some really neat shows you don't want to miss - the bird show, Nemo the musical, and the Lion King Experience. Plus, if you're there in summer, it will get you out of the heat. Again, go at your kid's pace. If you want to do a water park, I suggest Typhoon Lagoon - it's less crowded and more kid friendly. Go early to get a spot in the shade though - the sand is scortching!
Now to the real tips.... we brought two bags into the parks with us. I have an oversized purse that we put a first aid kit in, tylenol, chewable pepto, as well as neosporin, itch cream, bandaids and moleskin. Luckily, we didn't really need it, but it was reassuring to have. I also carried a couple of full water bottles (there are plenty of fountains at the park to refill), Noxema face wipes, and park maps. That's about all I took in that bag. DH took one of those drawstring backpacks with a hand towel and sunscreen. He also had one of those watertight containers that's just big enough for park tickets, id, and the bank card and kept it around his neck. We used this to store fast passes in too to make sure they didn't get lost. We did lose the drawstring bag at one point in MK but someone turned it in.
If you're staying somewhere laundry facilities are available, bring a roll of quarters, some Purell 3-1 sheets, and pack light. It's much, much easier to keep track of everything when you don't have much. Also, those purell sheets are wonderful - throw one in the washer, transfer with it to the dryer, and clothes come out clean and lightly scented.
If you're staying at a resort, get the refillable mugs the first day and learn where the refill stations are - they aren't good in the parks but make it easy to go grab a coffee or lemonaid or coke products. They even had GOOD teabags available at our resort.
For us, NOT planning the days at the parks and just exploring worked well, but it wouldn't work with older children who HAD to ride certain rides. The last time my IL's went with DH and BIL as teens, they planned out the day to the second to take advantage of everything they wanted to do and it worked for them.
Try to be in tune with your kids. If they are cranky, try to figure out why. Tired? Hungry? Dehydrated? Over-excited? A well timed snack break or ride on a train for tired feet made all the difference to us, but they were having trouble articulating their needs so it took extra patience to figure out what the best solution was. I know that sounds so basic, but with all the excitement it seemed easy for them to get out of sorts but not really know why.
Keeping my kids happy made the vacation so much fun. If they were relaxed and happy, so was I. I found I personally didn't really need much to make it an enjoyable, happy time. I did go to the spa the day before we left to get in some me time though - I think that went a long way toward me not feeling cranky that I didn't get any time to myself.
Have fun and if you have any questions, feel free to contact me!
D.