I would definitely push out the trip.
I'm a big supporter of babes at Disney---but when they are younger than 6 weeks..the risk is to high.
Factors to consider:
*Her Due date--it isn't a precise science--due dates are estimated calculations--she could be 2 weeks early--she could be 2 weeks late. You are in a tight window of opportunity--and dare I say...people have inductions for all sorts of reasons--an induction to make the trip...too tempting and unnecessary.
*Post Partum recovery---without getting graphic--I used the rinse bottle for the entire 6 weeks until post partum check up. She needs to take care of her own healing. A women having a baby--though completely a natural and healthy thing--her body still has gone through major trauma. So her health, in addition to baby's health is an issue. Also--what if she has a c-section? I'm not familiar with recoveries--but a jaunt to the mall or to the park is not equatable to going on vacation to a theme park when you are recovering from childbirth--either way the baby is born. She should ask her Obstetrician to see if there is even the possibility of being cleared to go on the trip.
And if you still want to go....
*Time of year---cooler/colder weather would be better b/c baby could be bundeled---May-September with a newbie newborn--they need to be swaddled/bundled and it just won't happen at this time. If baby needs this comfort---mom may be spending her time in the hotel room
*Travel distance--I would assume you aren't in the state of Florida--the further you travel--the further stress to baby and new mommy.
Sorry to be a bummer--I know you have it planned for a year...when is your trip taking place. She will be under a doctor's care--so surely, perhaps--
trip insurance would even allow a reschedule.
Good luck!