Yay Jess!! -/ That is fantastic! Looks like surprise trips are all the go for 2013
We went to Tokyo Disneyland with Ben and Jared when they were 3 (just turned 3) and 16 months. It was hard work but a LOT of fun. The boys will be 2, 4 and 6 when we go next year and I'm only really worried about Josh - the 2yo - for the flight. Ben and Jared will be able to entertain themselves for a lot of it (thank you iPad!) and hopefully sleep for a bit of it as well. Josh will not. So for you, I'm sure Matilda will be able to keep herself mostly amused, but not so much with the baby (sorry, I can't remember her name!).
As for the parks, I'm not at all worried. All Disney parks are very well equipped for little kids. When we were in Tokyo we used the Parents Room a couple of times, but there were change tables in all of the toilets that I went to, and even mini toilets and urinals for Ben
We went to the parks in the morning for up to five hours and then went back to our room for a rest before hitting the parks again. It worked wonderfully.
I think as long as you go prepared and well planned, but with flexibility, you will be fine. It was a COMPLETE change for Craig and I and our commando-style park touring is now a thing of the past. If your kids are a little timid, you might want to watch some YouTube videos of some of the rides to help prepare them, but it depends on the kids themselves. Ben was really anxious about going on rides, Jared loved every second and lapped up the Disney magic.
I'm so excited for you Jess, we don't know when we are going yet; March and May are the most likely times and I'm already planning away
-/ Nothing beats seeing that Disney magic totally enrapture your little one