Congratuations!!!!!!!!!!!!!!!
We went to Disney when I was 32 weeks pregnant (1st week of FEB, baby was born last week of March)
I was really sick for most of the pregnancy, however I felt the best the week we were at Disney. We took it easy and it turns out it was down right COLD that week. It was disapointing since I was hoping to get a winter break from Michigan, but I think it helped alot. I didn't get over heated or have any swelling at all.
It can be HOT in May. Can up push it up at all to late Feb or March? (I know you have older kids, any school breaks?)
Also when you make the plans make sure you can post-pone them with out much loss, that way if the time comes and you are not doing well you can put if off until the baby is born.
You deserve a nice break, don't worry you won't be the only pregnant women there.
Make sure to take it easy, take lots of breaks and try to be off your feet as much as possible.