Ditto everything Talent 312 said. I also think that having a "plan" with a 4 and 6 year old is necessary. Map out where you want to go before you get there. What are the "can't misses?" Just having a plan means you are not standing with a thousand other people looking at the map. It also means you can avoid backtracking. Years ago I bought a book called something like The Unofficial Guide to Disneyworld that had actual itineraries. My husband scoffed but it worked like a champ. Maybe still out there. When our daughter was five and our son ten, they were thrilled to go to the park in the morning and the pool in the afternoon. Throw in a round of mini-golf, a movie or something else and they were happy as clams.
It is really hot and really muggy in July for us New Englanders but we did survive nicely. We brought frozen water bottles - filled the bottle half full the night before, added more cold water in the morning. Stayed cold most of the morning. We brought snacks that didn't melt and splurged on popsicles or ice cream. We also went to lunch in the park - early - and got our fill of Disney magic. Then we went to the pool/waterslide/movies/mini-golf and it made for a full and happy day.
Universal wasn't a park then but we went when our daughter was 14 but it was in February, our odd New Hampshire vacation, and pretty uncrowded. Perhaps there is someone out there with more info on that.
Have a good time!
