I would vote for newborn.
I went when 4 mos pregnant and it was really hot (Sept) and I was still feeling sick. I went again at 8 mos. pregnant and felt like I couldn't keep up with everyone. Plus, both times I couldn't ride the rides I love the most. We went at the end of March when ds was 5 weeks old and dd was almost 3. My husband's parents went as well which helped since we could ride some adult rides while they watched the kids and they also helped out more with dd when I needed to feed the baby. As far as taking a child, it was the easiest trip with ds. Since I nursed him, feeding him was super easy, no bottles to carry. He mainly stayed in the snugli the whole time with my husband taking care of shuttling dd (his shoulders). I tucked a light weight blanket around and over him to block out wind and sun. And, he slept through the night for the first time while on that trip, although I did end up just putting him in his infant carrier instead of the crib at night. Now ds is 2 1/2 and it seems to take so much more effort to travel with him. We have to take the blankie or bear everywhere, plus snacks, cups, toys, etc. But we don't mind it too much. We'll be there again in Sept.