Our youngest DD was 7 months when we took her in January of this year, it was great! She did very well, would just nap in the stroller or sleep at night in the stroller if we were out later than she could hold out (which was pretty much every night since she would be asleep by 7pm). We had a great time taking her, and we just did whatever the kids wanted (most rides that my 3 year old could go on she could go on as well, so rides weren't really much of an issue). Any rides that she couldn't go on (like Dumbo), one of us stayed with her and hung out (I love people watching) while the other would go with my oldest.
It was a great trip and was not a problem at all! We've even taken her to Niagara Falls, Canada when she was 5 months old and she did great there too, it was cold as anything but we bundled her up and had a blast there too! We take her everywhere!