I'm just curious, how was the 9 month old visit?
Our son was 9 months on his first visit. It was a great trip. He really loved all of the music, swaying and clapping along to most of the rides. He also adored the characters (I think he was still too young to be intmidated by their size--his older sister who was almost three tended to hang back while he hugged every single one we met). We were surprised by how much he seemed to get out of the whole experience.
We went in October, so the heat wasn't bad. I reccommned trying for off season for the low crowds and cooler temps even more when traveling with such a little one. Also, use the baby care centers--they're great. I hope you get to go on you're trip--it's so much fun with a baby along.