My boys have been twice, at ages 1 and 3, and 2 and 4. they are both kind of timid on rides, and the older one is TERRIFIED of Pirates, hence no POC, much to his parents' dismay!!
The 3/4 year old loved:
At MK: Buzz, Indy Speedway, Teacups, Barnstormer, Aladdin (and water play area nearby), Dumbo, Philharmagic, Peter Pan
At MGM: Nothing! except the HISTK playground and playhouse Disney the first time
At AK: Dig site, KJ safari ride, Kali river rapids, Lion King show
At Epcot: Test track, Spaceship Earth, water play in area betwen the two sides of the park, meeting Pooh and friends in U.K., kidcot stations, Mexico rio de tempo ride, Norway ride, street shows.
The 1/2 year old loved:
At MK: teacups, tomorrowland transit authority, Pooh, small world, Buzz, dumbo, aladdin, Philharmagic
At MGM: Playhouse Disney
At AK: Digsite/boneyard, various trails, Lion King show
At Epcot: liked walking around, doing kidcots, water play in fountain, watching street shows, Mexico ride
We will not be going back to MGM on our March 05 trip, but they loved the other parks. they also loved the Universal parks.