We took my son when he was 25 months old and he had a great time!!! He got his face painted like a pirate and enjoyed getting the candy etc... We actually skipped the parade in favor of catching some rides. It was a great time to ride some rides with short waits and no sun!!
We took an afternoon nap (essential) but still only stayed until 10:00 or 10:30
We had a PS for Crystal Palace at around 5:30 or so that way we were fed and ready to go when the party started!!
Take him and have a great time!!! Be sure to get the picture (there was no line at the one near Liberty Square) - it's fun to get in the mail when you get home. Also, buy a picture frame - they're dated, but they run out so buy early.