Hello! We spent 12/29 to 1/3 at Disney with our sons who are 3 1/2 and 21 months. My biggest piece of advice would be to stay onsite if at all possible!!! We're going back in May and my husband had been saying "maybe we should save some money by staying offsite but nearby." We spent this trip with my parents in their RV at a campground about 10 minutes away and after the first day he was saying "I don't care how much more it costs to stay here...it's worth twice that with little kids!" Keep in mind that even if a hotel is a 10 minute drive away, you still have to leave the park, if at the MK (where you will likely spend most of your time with little ones) you have to get to the TTC, then wait for a tram and ride to your car. From the MK itself to our camper 10 minutes away took us about 45 minutes. If you are onsite it is MUCH easier to take a break midday for the little ones to nap in peace you also don't have to deal with your stroller on the trams!
The other big piece of advice is to just be flexible and let the kids guide you! Don't try to see absolutely everything unless you have a very extended vacation planned. Also, go early! You can get SO much more done in the mornings if you are there early before the crowds arrive. Early morning, midday nap/swim break, back in the parks in the late afternoon/early evening is the best way to avoid the crowds and long lines.
Must see attractions? Do a character breakfast. My kids are Pooh fans so we had breakfast with Pooh and they LOVED it. I cried watching my 21 month old literally RUN into Pooh's arms.

My 3 year old loved riding Dumbo. The Pooh ride and the Buzz Lightyear ride were big hits as well. They also loved the Country Bear Jamboree show. Don't miss Playhouse Disney at MGM. My kids danced through the whole show. There are nice playgrounds at all the parks. My kids loved having some free time (read, nonstroller time) to run around. Ask to ride in the front of the monorail. The ride from the TTC to Epcot is a good one, nice scenic views of Epcot as you arrive. Go to DTD and play in the "dancing water" fountains.
Some good planning tools are to get WDW With Kids (by, I think, Kathleen Willey) and the Unofficial Guide to WDW. We also used the electronic planner at Tour Guide Mike (
www.tourguidemike.com) and I highly recommend that as well! We followed his advice and had minimal wait times at everything even going on the busiest week of the year. The site can be a little tough to navigate through at first but he has GREAT info.
Other than that, the only thing I can think of is to have pullups for your 3 year old *just in case.* Mine always seemed to say "I hafta pee pee" at a time when there was no immediate bathroom availble. Changing him into a pull up saved us a few times! I also wish someone had told me that it is next to impossible for kids this age to eat a Mickey bar without knocking the ice cream off the stick before they even at the ears!
We had an absolute ball and I'm sure you will as well! It's so much fun to see everything through the kids eyes at this age. It makes the magic extra special!
(I have 4 different threads for our trip report in the trip reports forum if you'd like to read how things went for us!)