This thread is full of rants and raves about how much people's little ones loved Disney. I just wanted to say that the Magic Kingdom is not necessarily going to be a hit with all toddlers. You know your own kids, though, so I think you as a parent can make a good choice.
My niece and nephew adored the place when they were toddlers
We are probably outliers, but my son, on the other hand, at age two didn't want to go on any rides and felt scared and overwhelmed by almost everything at the MK. We thought that would be the case, but tried the park anyway to go along with extended family at Christmas time when we all met in the Orlando area. Note, too, though, that he is adopted from Russia, and this reaction to certain stimulating places is not uncommon with young toddlers/children who spent the first year of life at a Russian orphanage. We split up with the rest of the extended family group who were doing everything. My son liked looking at the castle, checking out the little area with Micky and Minnie's house, and watching Dumbo go up and down. He didn't want to ride Dumbo, though. I made him go on It's a Small World, and he found that to be really scary / feaky (bad choice). Anyway after two and a half hours, we headed back to our condo and the hotel swimming pool (a huge hit for our toddler and most toddlers). He was so happy to leave. We just did one other theme park that week (SeaWorld), and unlike the Magic Kingdom that was a complete bust, he just loved this place. He has always, though, loved marine mammals and fish.
My happy dance report though is that he has grown up to be a wonderful well adjusted teenager, and (so much fun for me) now even shares his mother's love for Orlando and Disney. Sea World is still his favorite park (I adore that place too), and I even send him to the SeaWorld overnight camp for one week each summer.