Just got back with a 3 & 4 year old:
MK - loved fantasyland almost all rides but esp. dumbo, teacups, and carousel. Also loved Alladins carpets, the race cars near tommorowland, tiki birds show, country bear jamboree. Alot here but centralized around Fantasyland
AK - animal safari, lion king show (awesome!), bird show, alot of seeing animals here, not many rides for toddler age children - Not alot of rides here period. This is more of a zoo atmosphere than theme park.
Epcot - Kid Cot stations, both kids loved Innoventions buildings very hands on interactive for the kids (called them the toy buildings), alot of walking around here and takes a little bit to keep kids involved. We purchased the passport books so the kids would have something to occupy them and between the masks at the kid cot stations and the passports (done at same time) it gave the kids something to do and look forward to at each country. Alot of people report their kids being bored in Epcot but ours actually really enjoyed it.
MGM - We (dh & I) also thought it might not be worth our time but again both kids really enjoyed this park. Playhouse Disney, Ariels Show were big hits for our 2. Remember that between walking around, character meetings (which there are lines for) and browsing shops take quite a bit of time so you may get to less than expected with a toddler in tow. We arrived at MGM for around 12:30-1:00PM and stayed till closing. I think it would have been too much to do opening to close in this park as there isn't a ton of things to keep you occupied ALL day long but still plenty to do. We started in Fantasyland at MK (we had castle reservations for buffet breakfast at 8:30) hit Dumbo, and a couple rides then headed over to MGM for afternoon. This gave us enough time to hit some shows, watch the parade, head over to Prime Time for dinner and get to fantasmic theater for the show there. Between the time for each show and walking around it was relaxing enough so we weren't stressed but we felt that the kids got quite a bit out of it. We also took some time for the kids to let off some steam and energy in the kids playground - "Honey I shrunk the Kids" playground which our kids always had fun in all the playgrounds at MK and MGM. More than I thought for this age group at MGM.