We've gone the last three years; twice the week after Thanksgiving and once the week after New Year's Day.
Some quick tips.
Character meals. Book early. Reservations open 180 days out. Akershus over Cinderella's Royal Table (we did both at breakfast; Akershus better and cheaper. Same princesses.). We like Crystal Palace (Pooh and friends) and Hollywood and Vine (Little Einsteins, Handy Manny, Oso). I'd skip Chef Mickey's. Too many other places to see Mickey and friends.
When to eat. I really like to schedule a sit down meal in the park either around 11 or at 1:30 or 2. It's really nice to get off your feet for a while, have a good meal, and give your kid a break. If you do it early or late, the wait will be shorter (your reservation is not a real reservation, more like a suggestion), and eating early or late will allow you to be in the park while the biggest percentage of people are sitting down to eat.
Where to eat. Of the sit down places we've been, I like Portobello the best. Unfortunately, we go to a lot of character buffets, none of which are all that great. I guess Crystal Palace and Tusker House are pretty good. The breakfast at Akershus is good. We ate at Les Chefs de Paris in Epcot, and I liked that pretty well. I've eaten at Ohana before, but I don't really remember what I thought. We usually stay at the Wilderness Lodge because you can take the ferry to Magic Kingdom, but I don't like the food at the Whispering Canyon Cafe, and the other sit down (Artist Point) isn't really appropriate for little ones. The place we stayed last year (Caribbean Beach) wasn't any better, though.
Buses. Since you're staying on resort, take them. So much easier than parking.
Extra Magic Hours. We're split on this. My wife likes them, but the EMH parks are always the busiest. The extra hour is nice, but if you go to a non-EMH park, lines will be shorter. The exception to me are special events like parades and shows (there's a Christmas party at Magic Kingdom and Hollywood Studios has a laser show, both after hours).
Park Hopper. Skip it. Especially since you're there so long, you'd just be wasting time on a bus.
Meal plan. We always do the middle one - 1 quick service meal, 1 sit down meal, and 1 snack every day. Incidentally, all the quick service meals count the same, even if one or more of the meal plans is for a child. We're 2 adults and a 5-yo, and the meals are not small. Plus, we do a lot of character meals, which tend to be buffets. Just one QS and one sit down per day is a lot of food.
Parades and characters. At the front of each park you can pick up a map and a flier that has the times and locations for parades and character appearances. You'll definitely want to know when the parades are whether you're watching them or not, since you can get stuck in an area for a while.
Rides. If you are willing to separate from your group, you can get in the single rider line for Rock 'n' Roller Coaster, Expedition Everest, and Test Track. I think RnR and EE are the two best rides on the resort, so I do this a lot. Usually my wife takes my daughter to the Beauty and the Beast show at Hollywood Studios and I ride RnR 4 or 5 times while they're in the show. At Animal Kingdom, EE is right up the way from the Nemo show, so we do the same thing there.
Stroller. If your 5yo is like my 5yo, he/she doesn't ride in a stroller anymore. Don't care. You're riding in it today, kiddo. Seriously, if I had just one piece of advice to give, this would be it. There's a lot of walking, and your kid will be a lot happier after getting pushed around all day. Plus, it gives you a place to stash stuff and tie balloons to. You might think it's a pain to carry around, but it's a lot less of a pain than carrying your kid on your shoulders or in your arms. I'm talking about a good
umbrella stroller, by the way. Something you can easily fold up and put under a seat.
Water. Not a bad idea to either bring some bottled water with you or buy some when you hit the park. Another little thing that makes a big difference when your group is getting tired.
Camera. We've tried it both ways, with my nice digital SLR and a little pocket Nikon. The Nikon was better, even though the pictures weren't as good. Just less of a pain. I got one of
these thinking it would help, but it didn't help enough to make we want to carry it.
Stress. Don't. Surprisingly, lots of guests in the park are not nice. They're extremely worried about how long they're waiting in line or the next event on their itinerary. My wife is an optimizer when it comes to these trips, so we've started staying longer so that I can tell her not to worry about seeing and doing every little thing in the perfect order according to some tip site or guy on in a football forum on the internet. It can add up to a lot of needless stress. If you stop having fun, take a step back and reboot. Vacations are supposed to be fun.
There's some cool new stuff in Magic Kingdom. You'll have a good time.