I'd try to split your trip as others have suggested. Spend a few nights at a Disney resort (that way you get Extra Magic Hours, Magic Bands, etc) and a few nights at a Universal resort (for the FOTL pass). If you must do Sea World, do it during the Universal portion.
Tickets will be separate. You'll need Disney tickets for any Disney parks (Magic Kingdom, EPCOT, Hollywood Studios, Animal Kingdom) and can get tickets good for only one park per day or for multiple parks per day (park hopper). I'd probably do 4 days at WDW. Keep in mind that the price per day goes drastically down the more days you are there. You can get 5 days for just slightly more than 4 days.
For Universal, you can also either get tickets good for one park per day or for multiple parks. There is a new Harry Potter section opening at US this summer (the other HP section is at IOA). I'm not sure when it will open, but they are building a train (the Hogwarts Express, of course) that will connect the two sections, but you will have to have the tickets that allow you to get into both parks on the same day. I'm guessing that won't be open by Oct. Actually, in terms of the 2yo, the Dr Seuss section in IOA is definitely geared for little ones. I'd probably plan two days at Universal, including one day at US and one at IOA. Keep in mind that Universal is definitely geared for older kids, and there will be many rides your kids (even the 8yo unless he is really tall) cannot ride.
Then plan Sea World for the remaining day. I think you can get combo tickets with Universal and Sea World, but you'd have to research whether that makes sense for you. You can definitely get Sea World and Busch Gardens tickets, but I agree that I wouldn't try to squeeze in Busch Gardens. Have you been to
Legoland? That might be worth a trip.