We took my daughter for her first birthday, and she'll be going again shortly before her 2nd birthday.
We have also gone with my nephew when he was 2 and 3 years old. So I've seen a wide range of behaviors.
The big thing is to let them dictate the pace, and put them on rides they will enjoy. My daughter is a big Peter Pan fan, and I know she'll go crazy when we go on Peter Pan's flight, even though it's not a ride I would stand in line for if just my wife and myself were going. We also rode Dumbo for the first time in many years last year, and really enjoyed it.
We always left in the morning after eating breakfast and returned back to the hotel in the early afternoon, where we had lunch. Both kids seemed to do much eating when they were out of their strollers. We would either eat in the PC food court, or by the swimming pool. We often get fruit, so they have something to much on between jumps into the pool.
If kids are hungry, they will eat, and if they are not - they will not. They will, however drink like crazy because of the constant warm temperatures. Make sure you give them plenty of cold drinks of water, juice, etc as the day goes on.
Also - the baby changing stations are a huge bonus, not just for changes, but for short breaks. I know the one in the MK has a TV to watch cartoons for a little bit and unwind out of the stroller. It also is air conditioned and usually quiet (I was often the only person in there).
That's what this forum is for folks! Let's get together as parents and share tips!