First of all, IMHO it's not a good idea to expose a young infant to that many people/germs.
Next, the baby will cry at night to be fed, and wake up you, your family, and potentially the people in the next room. (DS is 15, but I still wake up instantly when I hear a baby cry--when my neice/nephews were young and visiting, I got no sleep...)
On the same train of thought--you will be exhausted, and an exhausted mom with kids who are hyper to go to the theme parks does not a good mix make. Your husband will be tired from his conference, so I wouldn't count on much help for him, although the 9 yo is probably a good helper...but not enough to be alone with three kids in a theme park.
The amount of baby gear you'd have to lug is incredible! If you are nursing you'll have to be prepared for either doing so in public, or taking what can be a fairly long trip to the other side of the park to the first aid areas for privacy. If you are feeding formula, sterilization, heating and etc. will be at the least, problematic.
I strongly suggest you wait until you are better rested, the baby is older, and your husband can give you full help and attention, and plan a trip that way.
Anne