You are going to have so much fun!
We went last April (great time of year weather and crowd wise) with our 9 year old, 2 year old and 8 month old daughters and were anticipating many melt-downs but had NONE!
I researched alot, learned all about my favorite features fastpass and babyswap and talked to the kids beforehand about the characters (also received a Fairy Godmailer Postcard - that was AWESOME).
As did the pp, I also booked character meals - Pooh Breakfast at Crystal Palace (our favorite memory of the entire vacation - for your kids ages I suggest you do at least this one), Princess Storybook Lunch in Norway (to which my 2 year old wore her Sleeping Beauty Princess dress - buy it here so much cheaper - and although I didn't enjoy the food too much the Princesses were so lovely and my middle daughter is a Princess fanatic so it was fun to watch and we went to breakfast with Donald, Mickey and friends at Animal Kingdom - a GREAT breakfast and alot of fun for my husband as the original characters were always his childhood favorites.
We visited Orlando for just 5 days (Florida for 9 days but visited Fort Lauderdale first) and WDW for just 3 as we didn't want to be overwhelmed (did downtown disney the first evening - kids loved to dance in the streets - and lounged the last day, did a little shopping and packed up to go home). It is alot of work to haul babies around all day and stress isn't wanted on vacation. So my advice, pace yourself - you can always plan for another WDW vacation in the future and see what you missed (that way you can hang out planning on the Disboards) - find out the locations of the baby care centres (sometimes the air-conditioning alone is a thrill) and make sure you always have a FastPass in hand! BTW, Our top 2 rides for the babies were Pooh and Small World.
Have Fun!