We went for the first time when our twins were two and our oldest was four. It was so magical for the kids! They really and truly believed the characters were real. We did Magic Kingdom 3 out of 5 days and hardly left Fantasyland and Toon town. I can't even tell you how many times we rode Winnie the Pooh, but it they smiled from ear to ear every single time. If we had tried to do the baby swap and do some of the headliner rides, I think the kids would have been miserable though. We also didn't try to eat sit down meals. Most two year olds have no interest in waiting for food or eating at a sit down meal. I think if you bring young kids, you need to make the trip all about them. There really isn't a bad age to take your kids to WDW. Good luck with your pregnancy.
We are really hoping for just this experience! DD is four and we leave at the end of the week. We are just planning on doing MK 90% of the time with a lunch in Norway with the princesses and dinner one night at 1900 Park Fare. The rest of the time she is in charge of what we do, when and how we do it. I told DH, my sister and her DH to be prepared to ride the tea cups 100 times in a row if we have too!
To me this is the perfect time because she will never be more into the princess phase than she is right at this moment in our lives.