We were there during those dates last year. Weather will be warm during the day, but you still can get those cool breezes in the evening.
We also took jackets to the parks, then stashed them in a locker so we didn't have to tote them all day long. Don't forget to stash disposable ponchos in your fanny pack or backpack; we had frequent rains during that time.
Crowds were light-to-medium; we LOVED being there for the FLower & Garden Festival. With FASTPASS, we hardly waited for anything longer than 10 minutes.