We go to Disney every other year, and started that when our oldest daughter was 3. Through the visits we have gone pregnant (well, me anyway!), with infants (one 9mo, one 10 mo on their first trips), with toddlers, kids, tweens, and this fall will be our first trip with a teenager (YEIKS, you want to talk scarry!!!) Every stage has wonderful points, and more difficult ones. Let me tell you, the wonderful ones FAR out-weigh the difficult ones. Yes, it can be difficult to manauver the double stroller, you have to allow more down time for the kids, stuff like that, but in the end when you get to see them experience Disney and collect all the wonderful memories (and photos) that you will, you will be soooooooo happy that you did it. I would not trade any stage that I have taken my kids to. Sometimes I too would think "what are we doing, this is going to be hard", but once there, it was great, no regrets. I say go for it, but be relaxed and flexable. I always tell our friends planning their first trips and wanting to see it all that you can't. Unless you have monts to be down there (and un limited funds), you can't ever see it all, so just enjoy what you are doing, be greatful for it and when you get home, dream of what you want to do on your next trip. I would be stunned if you went on this trip and came back disapointed that you did--- it will be magical and I am sure you will be thrilled taht you went....
Have a great trip and take lots of photos, they will love seeing them someday (for too soon, time flies quickly!)