Crazy day here, but I'm back. Restaurants. I don't do DDP, so if some of these aren't on the plan somebody make sure to point that out... okay?
MK -
For a character meal, my favorite is the Crystal Palace. You can have any meal (breakfast, lunch or dinner) with Pooh, Piglet, Eeyore and Tigger. It is a buffet with a lot of choices. CS here that I like are Columbia Harbour House, Cosmic Rays, Pecos Bills and Aunt Pollys (but it probably won't be open in January). CS here that I would avoid are Pinocchio Village Haus or pretty much anywhere in Fantasyland. I have yet to eat good food in Fantasyland, and I am by no means a picky eater. TS I have enjoyed would be Tony's Town Square Restaurant and Liberty Tree Tavern.
Epcot -
For a character meal, we enjoyed the Garden Grill for lunch... oh, wait... didn't they stop the lunch with Mickey there? The food was good if you enjoy family style service. I don't know that I would eat there just for the food, since we were there mostly to see Mickey. CS here that I like are Sunshine Seasons (the best CS in WDW in my opinion), Liberty Inn, and the CS place in Norway has good sandwiches. The only CS here that I would avoid is the place in Japan. TS I have enjoyed would be Coral Reef (but it has been a couple of years since I've been), San Angel Inn (as much for the atmosphere as for the meal, lovely spot for lunch), and Le Cellier (but I'm not sure if Canadians would want to go to the Disney version of a Canadian steakhouse?).
DHS -
CS here that I like are Backlot Express and Studio Catering Company. CS here that I would avoid are ABC Commissary and Pizza Planet. TS I have enjoyed would be Sci Fi Dine In (another one for the atmosphere) and Brown Derby.
AK -
For a character meal, we enjoyed the breakfast at Tusker House with Donald, Daisy, Mickey and Goofy. It is a buffet with some standard and some unusual choices. CS here that I like are Flame Tree Barbeque (right up there with the Sunshine Seasons) and Restaurantosaurus (mainly because it is the only other CS that serves chicken, which is the only meat one of my sons will eat). I haven't eaten enough to have put anywhere on my "don't go there" list. The only TS meal I've ever had here was at Rainforest Cafe, which is a chain so it is the same no matter which one you go to.
I'm sure somebody can give you more/better lists, but that's mine.
I agree with much of Tricia's so I wanted to highlight some of them (good or bad!).
MK: I have to strongly agree with Crystal Palace (go if they at all can - the food is better at lunch than breakfast, which is just standard, but either is a lot of fun) and Columbia Harbour House (eat upstairs and get away from the chaos).
Epcot: Not sure how recently they stopped Mickey if they did at Garden Grill but we had a blast here last summer (07) and he was there then. The food is pretty good and it's nice that you can just sit and relax and not have to get up and walk back and forth to a buffet, but get all you want to eat.
I have to DISAGREE with not liking the CS place in Japan. This is, without a doubt, my kids' FAVORITE restaurant. When Stephen and I talked about going for free dining, he asked if we could eat there every day, twice a day if possible!

Now granted, you have to like that kind of food, but we loved the salads and the shrimp noodle bowl (which was a cross between soup and pasta).
Coral Reef we were very disappointed in the food for the cost. Just mediocre seafood for a lot of money. If you have really young kids, the view is neat, but really what you can see upstairs for free.
MGM: Brown Derby is a bit fancy and pricey, especially with young kids, and just keep in mind that it is two credits if you are on the dining plan. DH & I enjoyed it; I wouldn't take my kids there even now because of the cost. 50's Prime Time we really enjoyed and it's reasonable prices and food, plus fun. We enjoyed the Sci Fi (and if they've never been it's fun to do once for the atmosphere), but I wouldn't go again. Too loud, poor service, and hard to carry on a conversation because of how the "cars" are set up.
AK: I agree with Flame Tree being really good, and I also like Rainforest, but as Tricia said, they may have one near them. Our kids really liked that when they were young.
If they want to do ones out of the parks, some ideas would be Chef Mickey and Whispering Canyon. Really, though, making ADR's a few weeks out, they may be limited to what they can get. Give them the link to the menus and they can call, see what's available, and look at if they'd like to eat anything there:
http://allears.net/menu/menus.htm