We went at Easter, too, and had a fabulous fortnight - but I had planned it like a military operation! It didn't all go to plan, but definitely helped. DC are 14 & 12, so plenty of stamina!
The guide book I used was the Brit Guide to Orlando - which I bought in paper, then forgot to pack (oops!) so bought again for my Kindle when we got there! Lots of really useful information. I also used the park hours page on DIBB, which was good for planning to go to the different parks on the quieter days.
With regard to how many days, we had a couple each at Magic Kingdom and Epcot, one was plenty for Animal Kingdom, and we ended up going back to Hollywood Studios 3 times, which was a bit much but we needed to see Fantasmic and it was full the night we intended to go so we had to go back for it! We treated the water parks as "down" days.
Universal and Islands of Adventure both needed two days, but might not if you're staying in a Universal hotel and don't have to spend 2 hours queuing for Despicable Me 2! Our kids are big Harry Potter fans so that took a long time, too (really envy you getting to see Diagon Alley). Check out the Mardi Gras if you're there at Easter - free gig on Saturday nights (we missed Lynyrd Skynyrd but saw Kool & the Gang) and a fab parade.
Kennedy Space Centre needs a day, but one is enough - and it was DH's favourite day - highly recommended. We also went to LEGOLAND - one day was enough for that, too, but it was worth the trip.
Biggest tip is to get organised with the Disney Fast Pass Plus system - you can book 3 Fast Passes per day up to 30 days in advance, so you can make sure you get on the must-do rides without undue queuing. You need your ticket numbers to set up the account, but once that's done it's pretty straightforward. You can also book restaurant tables.
Have a fantastic time!