Weather and crowds that time of year are both pretty darn good. Weather is hot enough to swim, but lower humidity and less rain than summertime, and crowds are very low.
As to your boys, I couldn't tell you. It all depends on what kinds of stuff they like - E.T., Dora, Jimmy Neutron, Dr. Suess, Popeye, Marvel superheroes, Jurassic Park... there's lots to choose from that MIGHT appeal to them, depending on their personalities, interests, and attention spans. I recommend going to the Uni web site and browsing through the lists of attractions to see what your boys will like and what they won't.