We took my 3 year old to HDDR last summer and he enjoyed it for the most part. By the end, he was a little restless, but he's 3. I did some research before picking a seating level and chose to sit in the balcony so he could see better, which was a good idea. He was able to stand up and move around a bit, which made him happier. He liked the food and LOVED the strawberry shortcake. We also made a night of it at Fort Wilderness and after going to the early HDDR, went to the Campfire Singalong, which we really enjoyed. Just know getting anywhere in Fort Wilderness besides HDDR (it's near the boat dock...boats are best way to get there from a MK resort) can be a royal pain on their internal bus system.
Hope that helps!