Worth it for three hours on an Extra Magic Hour night after park closing? You bet. In the middle of a summer afternoon -- no way on God's green earth, and that goes TRIPLE if you're pregnant.
You're there for 4 days in summer, and your DH will only have 3 hours free during park hours the entire time? Are his meetings running until midnight?
In summer, night is the very best time for the MK. Take it easy during the day (you've got convention tickets, so mornings are out for you anyway.) Get the toddler well-rested in the daytime, and take him to the parks after 6 pm. It's OK if he stays up late in his stroller, he can sleep it off the next morning while Dad's at work. Do an EMH night if you can. Last trip my 21-mo rode everything she was tall enough for, and had a BLAST! (She was watching the planning video with DH today, and kept telling him about the things she liked. She likes Pirates and Buzz, but Small World, Philharmagic, Pooh and Dumbo are her very favorites. (We were at the Smithsonian last week, and when she saw their Dumbo car she got *so* excited!)
We just bring PJ's and a toothbrush to the park and get the littles ready to sleep before the fireworks. The minute that stroller leaves the park they go down for the count, but not a minute before; way too much wonder to take in.