From what i remember of Epcot, the World section was quite boring for me as a 15 year old, so I can only think that it would be even more so for a 7 and 8 year old. Am I right in thinking that the front half of Epcot would not take more than 1/2 a day?
I'm also remembering that DHS did not take that long to go thru, either.
If my recollections are based in reality, then we may have about 3- 3.5 days of non park time, which seems to me to be a fairly good ROI.
Any info on boat rentals? The idea of renting a boat and riding around the lagoon seems like a unique experience!
First of all, I agree that when I was a child, the "world" section of Epcot was boring. It's changed a bit since then, though. Don't write it off. Many kids love the World Showcase...they have KidCot stations at every country now, with arts and crafts reflective of the countries. There is also a World Passport that your kids can have stamped in each "country". There are rides in Mexico and Norway (boat rides...the Norway one is pretty cool for 7/8 year olds) and a lot of interesting things to see that you don't come across in your everyday. There is an awesome model train setup in Germany as well. You should plan to dedicate at least half a day for World Showcase. It's something your kids should experience, and you may be surprised by their reaction to it.
DHS is also a LOT different than your last visit there. However, I agree that for the 7/8 year old set, there's not much. I would recommend going in the afternoon/evening and making time for Fantasmic, at least. Toy Story Midway Mania will be opening this spring, and your kids will probably like that. If they are into thrills, there is the Tower of Terror and Rock and Roller Coaster (although, they may not be up for those). There is also the Star Tours/Ewok Village/AT-AT model if your kids are into Star Wars. I would take a look at the attractions/shows offered there and figure out, as a family, what you are interested in seeing.
I would allocate maybe one day to DHS/World Showcase (half a day for each), and another FULL DAY to Future World in Epcot (there is a LOT in Future World to see). You need at LEAST half a day at Animal Kingdom (just due to how large the park is...takes a long time to get around there) and then I'd do 2 full days at MK.
So, realistically, if this is all "new" to your kids, you need a good 4 days, preferably 5 to explore the major parks. This will vary depending on when you are going/crowd conditions/operating hours.
I would not count on 3 whole days of "off" time at the resort. Unless you are going for over 10 days. Rather than planning full "off" days, get park hoppers for as many days as you will have on property, but then only go for the morning or only the evening on some days, and spend the remainder of the time at the resort. Every other day, sleep in or plan to do the parks for only half the day. Every other day, be at the parks at opening and then come back for a midday break, then head back to a different park in the evening. This seems to be the best way to assure that everyone is not overwhelmed, too tired, etc.
Renting the boats on the Seven Seas Lagoon is FUN! I'd definitely recommend it! I like the Sea Raycers. You'd need 2, as they are only 2 passenger boats, and your kids aren't old enough to drive themselves. These boats are really fun and you can cover a lot of ground in 30 min. You can go over to Bay Lake with them (via the water bridge that the ferries to WL use) and zip around by Wilderness Lodge and Contemporary! It's awesome. Be sure to take your camera with you for some unique photo opportunites.
We've stayed at all levels of WDW resorts (values to GF, and we used to own DVC, too), and in our family's opinion, the Poly offers so much (all the things you mentioned, plus a relaxing, spacious room to come "home" to at night) that we definitely feel we get our money's worth.
go up, sometime in November, before Thanksgiving.
I was thinking about going to the AK Lodge for dinner, but deciding against it because of transportation issues, as well as my limited capacity to deal with them. 
DH and I were disappointed b/c we did feel rush and the resort was crowded. There were no quite places to just sit and relax and chill out from the park rush besides the pool/food court/arcade (which we always crowed)....so there really was nothing else to do. 
Had to sub... on our last trip, we decided that the Poly is our home 
