Since you are goiing over a fairly busy time, I'd say about the biggest potential mistake you could make would be to spend all of your time in line. Avoid that on ethign and you'll likely be fine. You should be able to do fastpass attractions with fastpasses, but maybe skip some of the attractions like Dumbo that aren't FP but do have overly long lines.
I'd also avoid doing the kiddie rides at 'kiddie' times of day if you can. If the four year old is a flexible kid - try saving IaSW until after 9pm.
WDW has LOTS of very fun things to do that don't involve lines. One HUGE hit in our family has always been the water fountains outside Capt Eo in Epcot. (you have to go up the stairs). Another hit is the landscaping. There's no line to see the amazing topiaries. One of the biggest impressions of WDW that remains from my childhood was the topiaries that used to line the monorail route. Simply put, they were unique to WDW. I went to other amusement parks, but WDW was teh only place I saw topiaries. WDW has all kinds of neat stuff to see, and CM's will be out in full force. Meaning soft attractions like the safari trail guide CM's will be out. Take a moment to see what critters they are showing.
Another big mistake you could make is to pull them too early from something they love. I once made that mistake myself. The kids I took were dying to hit the pool. I talked them into going to apark instead the first day. They enjoyed the park, but their faces really lit up when they hit the pool! (Something I consder very secondary to the parks.) In retrospect, I'm sure they would have loved the parks even more if they had been allowed to swim first.
Do not necessarily go to MK first. Probbaly go to Ecpot on Thanksgiving day, avoid MK on Saturday. Dodging crowds is still very important. Crowds can ruin a first trip. The important thing is to let them have fun - not to do things in the order of 'imporantce.' or to do EVERY attraction. I think that's a mistake many visitors make trip after trip. If you are feeling tired or overwhlemed or burned out by crowds - take a break.
Last, we're not big parade people - but I would make certain to take them to see the nighttime parade at MK! Again, of all parades, MK's is unique. They may not be old enough to appreciate the complex computer programs that go into make the allhe lights twinkle in sync, but they will love the effect. Then again just seeing MK at night is AMAZING! Likewise, if you have the chance to see the Osbourne lights, GO! GO, and get a cup of hot cocoa while you're there! (especially if you can go while it's 'snowing'!)
(one of my favorite things about MK at night is seeing how all the lights twinkle. Gaslights look like real flames, the treelights in Fantasyland look like magic fireflies!)