MK is awesome for young children and almost everything is pretty exciting to them there. You only have a few height restricted attractions and not a whole lot of "scary".
My then 2 year old loved dumbo, aladdin's carpets, small world, Peter Pan, little mermaid, the carousel, Winnie the Pooh, the tomorrowland speedway, and monsters laugh floor. We did make sure to try ride type things before we went (carousel at our local zoo) just so it wasn't a completely foreign concept to her.
I could easily spend multiple days at MK with a toddler/preschooler.
We did some Epcot, but oddly fared pretty well at Hollywood studios. My daughter refused to go see nemo or crush at Epcot last time, but now she's almost 5 and pretty excited about those. Then she loved the Disney junior show at studios at 2. I think that was all we did, but she now is going to love the frozen show, TSMM, and potentially beauty and the beast if we have time.
Particularly if your child likes characters, there's something to do in each park that can be unique as well.