Disney's new ticket pricing structure might be PERFECT for you guys with so many days.....There is a thread here somewhere that explains it,but you can actually get 10 days for something like $28 a day using this new pricinig....'the longer you stay, the less pay' seems to be the new strategy.
It always depends on the crowds and the durability of those in the party (when my d was 3 she needed two naps a day)....and whether you are staying onsite (which makes going back to the hotel midday a nice alternative). And the hours the park is open too.
That being said, to do EVERYTHING in each park, you'd need the longest time in Epcot, then MK, then MGM, then Animal Kingdom. Personally, I like to mix up the parks, especially when MK is open til midnight...I might do AK until 4, take a break and go to the MK for the night, fireworks. Or AK and then MGM for afternoon and Fantasmic.
If you are a first timer, it is awesome that you will have so many days and can stop and smell the roses (or Hidden Mickeys)
And spring is a lovely time to visit!
Have a blast.
Colorado Bell