I'm going to go with Small World. I know some adults don't care for it (personally, I love it), but from the eyes of 4 or 5 year olds, I think it's a great Disney place to start. Not that I think any of the other ones are "bad", I'd have no problem taking kids that age on those rides. But to me, IASM doesn't have a ton of high tech lights, lasers, noises, etc.... It's lots of simple moving animatronics (if that's the right word for them), almost like cartoon characters, with simple music playing. I realize Pooh and Pan are fairly simple too, but my preference would be IASM simply because of how big it is and how much variety it has.
It could also be personal bias, as it's one of the few rides I remember from my first visit to WDW, when I was pretty young. In my eyes, it's as Disney as WDW gets.