<font face="Comic Sans MS">How exciting! You'll have a wonderful time!
We took our first trip last October when our DD was 19-months-old. As Poof mentioned, the water fountains are great entertainment! As for water in MK, try Ariel's Grotto in Fantasyland.
We did early entry at all the parks, then returned to our hotels after lunch for some down time. I think we enjoyed this part more than our DD did. We usually napped for 2-3 hours in the afternoon and returned to the parks in time for dinner. The parks have shorter hours in the fall, but they're also less crowded so you'll have plenty of time to see everything you want without running yourself ragged.
If you're using a stroller, it sometimes helps to have some sort of identification on it (we used a bandanna). CM's will sometimes move your stroller from where you've parked it. Having something to make yours stand out from the others might save some time.
I second the recommendation for dining at Chef Mickey's. You can't beat the atmosphere--your kids will love it! We arrived at WDW in the evening and had our first taste of Disney with Chef Mickey. It's still my favorite memory! Another great character meal is Crystal Palace in the MK. It's very laid back, and the characters were wonderful! If your girls like the princesses, it's worth getting up early to get PS for the character breakfast at Cinderella's Royal Table.
If you're in the MK for the fireworks, make sure you catch Tinkerbell flying from the castle. If you'd prefer not to fight the crowds, though, you can catch a great view of the fireworks from the California Grill's balcony at the Contemporary.
Sorry this got so long-winded. I could go on and on, but I'll let others chime in with their tips. Have a wonderful time!</font>
