Not sure what time you are arriving, but the concensus seems to be to check in as early as possible at the hotel. Realize that you probably will not have a room until 4pm and plan accordingly. Our kiddies are almost the same ages as yours, and the part I am most dreading of our trip is that first full day when we probably will not have a room for nap time. Hopefully they'll do OK dozing in the stroller.
Our plan is to at least maximize that first day by getting through the Animal Kingdom. We'll try to arrive early, and stay through the 4pm parade. By then our room will definitely be ready, making for the perfect time to leave the parks behind and take the evening to get settled.
Have fun!